Originally Posted by: NickFerra
One progression is C, Em, Am, Em with a capo on the third fret.[/QUOTE]

This one is pretty straight forward. C Major scale is going to work really well. If it makes it easier to get started, you can think of it as A minor pentatonic. That'll get you going... but try to incorporate the full major scale if you can.

You could also use C Lydian for this progression... the fourth degree of the scale is raised from F to F#... a little tricker to fit in there, but may give some phrases an interesting sound if used with care (be careful with Lydian scales, your guitar may explode).

By the way, if you're saying that you're using the chord shapes but transposed up with the capo on the third fret, then you're going to be in the key of D# / Eb and you need to transpose to that key.

[QUOTE=NickFerra]
and another one is D, G, Cadd9 for the intro and verses and C, D, G and Em for the chorus.

Ok, this is where my theory kind of sucks, but to my ear, it sounds like with those chords you're in the key of G Major. The G Major scale will work great with them. Again, you could think of this as E minor if you're more familiar with that scale. If you're only familiar with pentatonic scales, the E minor pentatonic will work... BUT, you're leaving out on F# and C, and those notes will really help with creating melodies, so learn the full diatonic scale if you can... staying in just one position so you don't get lost and then experimenting with the full scale, hearing how each note sounds over the chords is a good way to start.

Hope this helps!