Generally speaking:

Play a song in "C" shapes (C, F < G or G7) in the open and you are playing in the key of "C". Put your capo on the 4th fret and play those same "C" shapes and your are now playing in "E". But you have the bennefit of those nice bass runs that "C" provides.

"C" Shapes
capoed at 1st fret = Key of "C#"
at the 2nd fret = "D"
at the 3rd fret = "D#"
at the 4th fret = "E"

and so on.