Actually, it's much easier to play in any key than you might think. As soon as you can play an A Major barre chord, you can literally play any other barre chord on the fretboard. Just move the chord up or down any number of frets.

You'll then have to learn the chords that are actually in the A major key. There are only seven chords. But just like before, all of these chords move just like the A major chord.

So in order to play in every key, you just need to remember the pattern, which consists of 7 chords. Move that pattern up and down and you're playing in every key.

Here are the seven chords (but in C Major):

C Major
D Minor
E Minor
F Major
G Major
A Minor
B Diminished

Now, just learn those barre chords. Again, all you have to do to play in C# Major is move ALL of the chords up one half step. To play in A Major, just move them all down 3 half steps.

Confused yet? ;) If you are, I can make explain better or in a different way so you can understand.