how do you figure out what key your in?

and do diffent chrods have differnt keys?
^you figure out by the chord progression. this requires some music theory knowledge.
And yes, some chords are only in some scales.
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the notes you play and the chords you play determine what key your in. i just choose a key to play in.

yeah every key has chords that fit in that key
find the major chords of the chorus, find the one that isnt right next to another in the musical alphabet. if the major chords were a, c, and d, the key would be A. or you can just find the chord it resolves in....
Try to play in that key(unknown), find out the note where u can sustain(most importantly, it should give a feeling of completion), thats the root tone. And then find out if its a major or a minor key.
if its just a simple song... You might start and finish on the same chord... usually that chord is the key.

also, look for any sharps or flats you play... that will help determine what key your in too... read up on key signatures
See what notes you're playing so you reach a possibility of two keys, one major and one minor though they're relative to each other. Then, figure out where your tonal center is and you'll have your key.