What scales should i use when im improvising major or minor on guitar?
If you're new to improv then learn some 12-bar-blues, it's the best to start learning on.
In a 12-bar blues, play the blues scale (minor pentatonic with an augmented 4th) over each chord as it is being played. If you are just playing in a minor key, play the natural minor scale or harmonic if you want it to sound egyptian (you'll have to modify the underlying chords so the #7 of the harmonic minor scale doesnt clash with the music)
Pwr gave good advice - it's a nice, basic place to start that can lead to a lot of other places. As far as improvising, you need to know what progression, scale, or note/chord key you're improving over to decide what scales to use.