how to know the key?

If you have sheet music, the key is indicated by the number of # or b marks next to the treble clef. It indicates the number of sharps (or flats) in a scale. http://en.wikipedia.org/wiki/Modern_musical_symbols#Key_signatures How do you know how many sharps or flats are in a scale? You have to learn the scales. The tools on this site are very useful for learning scales.

If you're trying to learn a song by ear, or without sheet music, for lots of popular music the first chord indicates the key. But this is not a strict rule by any means. There are many other tricks to figure out which scale works for the song you're trying to learn, lots of which rely on your knowledge of scales.

For a beginner, I would suggest learning the Major Pentatonic scale, and use it to play along with songs you want to learn. Just start the scale on the note that matches the most common chords in the song you're playing. This gives you a basic framework you can strike out from to learn more sophisticated playing.

sound it out brotha and then shred