Over every chord, there are so many scale choices to use, bebop, the modes, the melodic minor modes, the harmonic minor scale possibilities, pentatonics.... the list goes on.
But if you just go by that you won't get anywhere, because your playing will sound like a scale. It's important to learn to create melodic ideas in jazz, you can just sweep up and down different scale patterns (though I still admire the people who can do that).
You also have to learn how to flow these ideas into eachother, this is hard because most jazz tunes change keys every few bars, so you have to use a different motif.
Pick up some of the Jamey Aebersold books (there's a link to his site on my page... if you wanna check them out), listen to Wes Montgomerey, Charlie Christian, Pat Metheny.... etc. That will help you. Figure out some licks you hear in your head, and apply them.
Also, try some websites, such as jazzguitar.com... or search on google for other stuff.... eventually you'll be able to play smoothly in all types of situations.
Good Luck! Jazz is a hard artform to master, and I put jazz musicians at the top as far as overall musicianship, because they have to learn so many things and apply them. It's complex stuff man... it also helps to be able to read music up and down the neck, and to know what the chord symbols mean.
There are only two important things in life - There's music and theres girls, not necessarily in that order....
The Ace's Guitar Tricks