Start with a simple chord progression in C major: C major, A minor, D minor, G7. Then progressively add notes of the C major scale. Don’t think about notes. Think about guitar fingering (anyway, if you know the fretboard, you know what note you are playing too). If you add notes of the C Major scale, they will sound good. This happen because C major, A minor, D minor and G7 have the same notes of the C major scale (C, D, E, F, G, A, B). Let’s see :

C major = C, E and G

A minor = A, C and E

D minor = D, F and A

G7 = G, B and F

One of the ways to improvise a melody is to base your solo line on the notes of the underlying scale (as seen in the previous example). This type of scale-based improvisation is called horizontal improvisation. Another way to improvise a melody is to base your solo line on the notes of the chord (vertical improvisation). A chord based solo is a little like playing arpeggios. Anyway you could use more than one scale to develop your solo. Let’s take a look at the previous chord progression. There are certain scales that will magically fit this chord progression and allow you to improvise freely:

C major = major scale, lydian scale, pentatonic major scale
A minor = pentatonic minor scale, blues scale, dorian scale, phrygian
scale, aeolian scale
D minor = pentatonic minor scale, blues scale, dorian scale, phrygian scale, aeolian scale
G7 = mixolidian scale

If you like rock or blues guitar, you should take a look at pentatonic guitar scales too.