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If I'm playing in a major scale, would a harmonic dissonance be caused by going to the minor side of the chords (ii, iii, vi) or would it be created from going outside my scale? And if it's going outside then are there any rules or do you just pick a few notes that sound interesting together. Or do you modulate to a different key? And is the melody involved, or does harmony mainly change for tension?
I'm trying to create tension/fear/uncertainty in some parts of an overall cheerful song.
I'm going to give you the answer to the question you are asking, but I highly doubt it's what you are looking for. Almost every harmonic topic in tonal music relies on the concept of consonance-dissonance and most concepts in music have at least some dissonance in it. Whether it accomplishes what you want or not is entirely different.
V7->I is a dissonance (tritone) resolving to a consonance. Any convincing modulation will have a dissonance resolve to the new key's tonic (even sudden modulations fulfill this). Small intervals in the bass almost always are perceived as dissonant (even intervals that we typically think are consonant like 3rds). Secondary dominants, Augmented 6ths, modal borrowing, Neapolitan chords were ways to use chromatism to propagate dissonance. Blues tends to add extra dissonance with making all chords a dominant 7th. Jazz adds extra dissonance with concepts like tritone substitutions and use of extended chords.
This patter continues for many different tonal styles and tastes and there are many different ways to approach this. The paragraph above barely scratches the surface and obviously each idea above produces a much different result. One other event I'd mention in the history of music is Schoenberg's emancipation of the dissonance which provides the basis of the the goal of atonal music.
What it sounds like is you want to provoke a certain emotion, but you don't know how to do it. How you provoke any emotion is tricky since what sound tension/fear/uncertainty will be perceived differently by everyone. What I recommend in this case is to find music that makes you feel that way and what about it makes it sound that way to you. One example I'll give you that at least causes this effect for me is Jaw's Main Theme show below. While harmony does help build tension in this song, I'd argue that the instrumentation, form, pacing, space, texture, range, and rhythm effect it just as much if not more. It does however contain one of the dissonances discussed above with is small intervals in the bass register.
Dissonance as a concept has two basic thoughts, one of which being a little more controversial. The first, which is the more commonly used, is that dissonance and consonance are basically relative, existing on a spectrum. The most consonant intervals and harmonies are those that have their overlapping sound wave cycles line up most frequently. An octave is the least dissonant interval, with each of the higher frequency sound cycles lining up with the lower frequency sound cycles 2:1, ie, each sound cycle of the lower frequency happens once in the same amount of time it takes for the higher frequency to happen twice, as pictured below. More dissonant intervals take more cycles for their sound waves to align. This explains why the same intervals tend to sound more dissonant in lower frequency ranges (if you're not familiar, try playing a major 3rd in the middle register of the piano, then toward the low end).
The other thought process for dissonance is the desire to resolve. This generally applies well to the first concept, as the most common approaches to music typically use dissonance as a means of setting up a resolution, eg, the V chord is used to create a tension that wants to resolve to the I chord. This only really becomes controversial when taken to the extreme. Atonal music is basically set up to make no notes more important than any others, so there is no sense of tonic. Because of this, there can be no desire to resolve to any given place, as no given place is any more at rest than another. In this sense, Atonal music is the most consonant music there is, which almost every human that has ever heard it would entirely disagree with.
Dissonance, as most commonly conceptualized, exists on a spectrum and is relative. This means that we can adapt to dissonances and begin to perceive them as more consonant. This is fairly evident within the world of Jazz. Extended chords can sound very dissonant when placed randomly within a piece of music but when the entire piece is made up of extended chords, they sound very consonant, especially compared to the altered chords that are often used to set them up.
With this in mind, it's important to consider your context when you attempt to use dissonance to create tension. Something that would sound fairly dissonant in a Pop song may very well sound entirely consonant within a Jazz song. Additionally, some intervals may sound dissonant on their own but when played within the context of a chord, sound much more consonant. A minor second sounds pretty dissonant on its own but when added to certain chords, it sounds very nice. For example, if you're constructing a minor 7 chord with some extensions, you can place the minor third and the nine directly next to each other, creating a minor second between them, and it can give you a very nice texture that sounds significantly less dissonant than the interval by itself.
One of the more difficult intervals to make sound consonant is the minor 9th. When learning to voice Jazz chords, one of the "rules" is that you should make sure to avoid minor 9ths on any non-dominant-functioning chord, as the dissonance subverts the function of the non-dominant chords. This is pretty well exemplified by creating a major 7 chord. If you voice the root note more than an octave above the 7, you get a minor 9th, which sounds less than at rest and within a major key, Imaj7 should sound fairly at rest. Flip those notes around in your voicing and all of a sudden, it sounds very nice and at rest (within that context). Because of this, when songs that end with the melodic note playing the tonic, the final chord will usually not be a maj7 chord, as it will almost always create the minor 9th (it will often be replaced with a maj6 chord).
So I think what you're looking for would be a mixture of using certain types of intervals/chords that are generally considered dissonant regardless of context (such as b9s, diminished chords, tone clusters) and setting up your dissonances within a given context (such as using strictly triadic harmony and interjecting more complex chords). Using notes from outside the key you're in can also cause dissonance as well but if you listen to enough Jazz, you'll find that playing outside isn't always inherently dissonant.
Beyond all that, there is also a sort of tension that can be derived from arrangement. Learning about the extended techniques of certain instruments can allow you to utilize them to create more tension from the same melodic/harmonic material. For example, on bowed string instruments, playing near the bridge, called Sul Ponticello (I usually just hear this referred to as Ponticello), makes a very scratchy sort of tone. This technique ultimately does create more dissonance, as it emphasizes all of the natural harmonics and with more harmonics being audible, there are more notes, which means more dissonance, as all of the sound waves are then being heard simultaneously (more sound waves needing to line up).
So you can basically create all different sorts of tension and dissonance through different approaches. There is no one way to do it and there aren't necessarily any rules. You're actually more likely to get tension by breaking the "rules", such as with my Jazz example of voicing the chords properly to avoid minor 9ths. As I often do, I'd like to clarify that theory is not a set of rules, it is a language to describe music and an explanation for why it sounds good. The only time there are "rules" are when you're trying to emulate something specific, like if you're trying to sound like Bach, you have to follow proper Classical voice leading rules.
With due respect, some of the questions you pose come from 'what part of theory has to get used to make this happen?' This one is in similar vein.
It's a slightly unhealthy approach to music generally, and composing in particular.
Things tend, in music, to work the opposite way. Find something that's effective, use it, then try to explain it away in terms that somehow make it plausible.
There are many 'tricks', but the best ones come from experimenting. I don't believe every composer thinks 'right, now I want something creepy, so I need to use xyz'. If they did, there would be a certain sameness for every creepy bit of music. But wait, yes, there probably is!
And one of the tricks, just like in story telling, is to give the listener what he's not expecting.
Waiting for a perfect cadence? Give them an interrupted one. Waiting for a nice mellow major sounding chord? Give them anything but. There probably is a list, but if you're the composer, experiment, and come up with your own ideas, otherwise it's going to sound somewhat clichéd...
A few different ideas for creating tension/dissonance:
Mathematical dissonance - although it's somewhat subjective, scientists over the years have concluded that dissonance is something that can be measured. So you could be aware of this 'measurable' perspective when you compose. As Dom has pointed out, you don't always have to go outside of a scale to do this - the leading note is there in the major scale partly because it allows you to create an interesting dissonance.
Defying expectations - no-one can give you a formula for this, because once there's a formula, it's not unexpected any more! But a chord progression that goes in an unexpected direction will create uncertainty.
Following expectations - If there's a musical device that is already culturally associated with the feeling you want to create, you could simply use it (as close as you can without getting into copyright trouble!) in the manner of 'commonly-understood vocabulary'. Example - the new Instagram superzoom...
Also, I know your question is about how to create harmonic tension, but it's always worth remembering that you can create tension using aspects of music other than harmony. Sometimes one note is enough, as at the start of For a few dollars more, or Flash Gordon - in fact the latter starts with no notes - the drum beat itself provides tension. The unpitched, distorted guitar 'smashes' in Creep provide a sense of anticipation too, and yet whatever is played there has no identifiable harmonic quality - it's just an aggressive noise.
Dissonance in harmony is, to some degree, due to a conflict in the harmonics between the two (or more) notes being played. There are dissonant intervals right in the diatonic scale, the major second and the major seventh for example. Dissonance is also generated by voicing. Rather than finding new notes to stick in a piece of music, play with the ordering of the ones you have. For example the interval of a minor second is generally considered very dissonant, in the major seventh chord you can stack notes to hear the 7th-8th tension. Move the seventh to the bass register.
For me, writing counterpoint exercises taught me a lot about dissonance. Introducing a note that is consonant, holding it and changing its harmonic surroundings -- thus rendering it dissonant -- and then resolving it, making it consonant.
I used Arnold Schönberg's "Preliminary Exercises in Counterpoint" which I heartily recommend.
