I can find basic chords for most of the "normal" melodies by ear, but I can't find right "Jazz" chords like 7th chords..(advanced chords). Well,I'm not bad in chord theory. what do I need to study in order to find advanced chords for a melody? Do I need to play more songs ? Do I need to study counterpoint ? Thanks in advance:)
I'll qualify my answer here by saying I've studied jazz theory more as an improviser than as a composer.
My first thought is that you should study some jazz tunes to learn a bit more about the kinds of harmony that are used. Learning a bit about chords with extensions would be helpful as well, but actually I consider that more a 'stylistic' addition than a really essential one to the core of the music—often as not, Gm, Gm7, Gm9 will mean essentially the same thing: the ii chord of a ii-V-I progression.
You'll find some tunes that can be understood fairly well from a classical standpoint, with the melody following fairly simple progressions (but moving over a variety of keys quickly). The first few bars of Miles Davis's "Solar" is a good example:
Chords: Cm | Gm7 C7 | Fmaj7 | Fm7 Bb7 | Ebmaj7... Analysis: i | ii V | I | ii V | I... Key: C minor | F major | ... | Eb major | ...
The movement by ii-V-I is nothing particularly "advanced", and while I've written the chords as they're often notated in jazz, you can understand and hear the harmony just as well ignoring the 7ths and such. Actually playing the tune, I may play the second and third bars as "Gm9 C13 Fmaj69", but all of those chords still come out of F major.
Another example, from Henry Mancini's "Days of Wine and Roses", and is a little trickier:
Chords: Fmaj7 | Eb7#11 | Am7 | D7b9 | Gm7... Analysis: I | bVII7 | iii (ii) | VI7 (V7) | ii (i)... Key: F major | ? | transitioning to G minor...
So, this one borrows from a sort of progression that's pretty common, namely a kind of walking down of dominant chords to an VI7 chord. In this case, however, Mancini's interposed an Am7 to function as a ii chord, really driving home the transition to Gm.
You'll find this kind of thing (a substitution, it is) done quite a lot in jazz. It's very common with vi chords, both in order to drive one's ear really solidly into the ii chord and to open up some harmonic possibilities. The dominant VI7 can be altered in a whole variety of ways (say, VI7#5, or VI7b9) to expand the possibilities for the improviser or composer.
And now that I look at it a little more deeply, you can even think of that bVII7 as a funky kind of substitution that's also pretty common in jazz, this one called a "tritone substitution". One of the best ways to get into that D7b9 with some style is from a fifth above, which is exactly what happens. But if you wanted to be a little tricky about it, you could substitute for that Am7 a chord a tritone (or diminished fifth or augmented fourth, as it suits you) away or an Eb chord. Here, Mancini's kinda flopped it around from the one to the other, in a way that works really nicely to my mind.
This is particularly common with dominant chords, because that tritone substitution keeps the relationship between the 3rd and the 7th note of the chord intact, simply switching them. In a jazz 12-bar blues type form, you'll hear a whole lot of that happening at the last turnaround. Where some would play a I-vi-ii-V type turnaround, you'll very often hear something like I7-bIII7-II7-bII7, because jazz players love dominant chords and tritones. As you can see, this is the sort of thing that compounds quickly, and for that reason I've offered only the briefest introduction, as there are indeed whole books written on the subject.
At any rate, if you fully analyze the melody to this tune, you'll find that Mancini hits chord tones really hard. In my mind, the melody follows the chords rather than the other way around. Now, his arrangement does some other cool things, and most jazz players will "stretch" the harmony around in other ways but the basic understanding, I think, comes from study of historical progressions like these and others.
If you really want to get serious about learning jazz theory, I'd recommend Mark Levine's most excellent jazz theory book, "The Jazz Theory Book". It's very much geared toward the performing and improvising musician, but it covers a lot of the weird progressions and substitutions that you may want to use in your compositions. I'd also get a good fake book (I use the Hal Leonard "Real Book" series a lot, and Sher Music's "New Real Books" are also very good) and pick as many of those tunes apart as you can until you start to understand some of the twists and turns a little better.
Look at the lead sheets for the melody and see what notes are being played with the chord. This can give you an idea of how advanced chords are built. If you are seeing complex chords with 9, 11, 13 extensions it is more than likely that the melody note is the 9,11,13 of that chord. For example a melody note Ab over G7 would then give you a G7b9 harmony.
What you should practice is ... 7th chords. Since you can figure out the basic chords, I guess you have studied them, right? Do the same thing with 7th chords. Start playing them or listening to them (on some software) and you'll eventually get used to them. Then you'll be able to find them.
In jazz, the majority of chords contains a 7th, so you could mess around with normal chords and normal chord progressions, like IV - V - I (or in a more jazzy way II- V - I) and then add the 7ths.
It would be good to take a simple jazz song like "Autumn Leaves" and "Blue Trane" both of which have simple harmony. It might help you with the 7th chords