The fact that you are a guitarist suggests the niftiest answer: the layout of the fingerboard is itself a great learning tool.
Play, say, an A major scale starting on the fat string at the 5th fret using the common fingering of 2,4 1,2,4 1, 3, 4. For now, just go one octave and finish on the 4th string.
Those 8 notes give you the 8 degrees of the scale: 12345678.
Keep your hand in position. The major triad will fall under your fingers: 2,1,5
(A C# E).
The names of the main chords in any key, I, IV and V, based on scale degrees 1, 4 and 5, can be memorised. I and IV are on the same fret, one string apart and V is on the same string as IV but two frets higher.
At first you will have to count up from known notes to name the notes, but you will start to memorise them in time.
The next step is to work out what notes are in the main triads, I, IV and V. Well, you already know triad I (notes 1,3 and 5 from A major scale; A C# E).
Stick your 2nd finger on degree 4 (5th string, 5th fret). That note (D) is the root note for chord IV (D major). It just so happens that you can use the same old major scale fingering pattern starting on D (5th string 5th fret) to find the notes of the D major scale. Notes 1, 3 and 5, falling under fingers 2, 1 and 4, are the notes for the D major triad.
You work out Chord V the same way. You know that the 5th degree of the a major scale falls under your 4th finger on the 5th string (7th fret).So now you need to play the major scale starting on that note. Shift your hand and put your second finger on that note (E, 5th string 7th fret). Now use the same old major scale fingering pattern, 2 4, 124, 134 to work out the notes in the E major scale and then pick notes 1, 3 and 5 to form the triad.
Hopefully you will come to recognise that fingers 2, 1 and 4 will always give you the notes for a major triad. This works on strings 6 and 5, 5 and 4 and 4 and 3.
It doesn't work for strings 3 and 2, but it works again for strings 2 and 1.
Hopefully you will also come to recognise that the root notes for chords I and IV are always on the same fret on adjacent strings and that V is two frets up the neck from IV.
This remains absolutely constant right up and down the neck for all keys, so if you can visualise it for A major you can visualise it for all keys.
As for working out what chords you can use, the following is a good starting point:
Chord I is major
Chord ii is minor (note the lower case roman numeral for 2 indicating that it will be a minor chord) but that rule is often broken and a dominant 7th chord substituted.
Chord iii is minor
Chord IV is major
Chord V is major or dominant 7th. In some cases, a minor seventh can sound cool too.
Chord vi is minor
Chord vii is diminished.
The major keys will keep you busy for a while, but as a taster, for minor keys chord i is minor, chord iv is minor and chord V is major (dominant 7).
So in summary:
Use the scale fingering pattern 24 124 134 to find the degrees of the major scale.
Use the pattern, 'I and IV same fret adjacent strings and V is two frets up' to name chords I,IV and V.
Use the same old fingering pattern, 24 124 134 again to find the notes in scale of Chord IV and again for Chord V, then use notes 1, 3 and 5 to for the triad for Chord IV then Chord V.
In this way your fingers will find the right notes long before your brain does. Plenty of people stop there, but once you've got used to using these patterns get into the habit of nutting out the note names and saying them out loud. You won't regret it.