playing chords with guitar

[2eyebipedal]

i played the piano for a while, but then quit because I got tired of playing classical music.
But then my brother took up the guitar and wants me to play again.
I found that if you take guitar tab you can change it into piano notes and it's not that bad sounding. untill u get to chords.
how do i find what key my brother is playing in, so then i can play chords along with that? is that how id do it? im all confused.

[donjuan]

how do i find what key my brother is playing in, so then i can play chords along with that?

ummmm..... ask him? plan it out?  Unless you have a decent amount of music theory and the talent of perfect pitch, you probably wont be able to hear him play something and then think "oh yeah, thats a C# 6th"