#1
How do you know what chords go in what key?
Fender Standard Telecaster
Kinman Pickups
D'Addario Strings
#2
I'm not gonna give a full answer to this as I just woke up and can't think straight [and I'd probably be somewhat incorrect knowing me], but there are formulas for working keys out [ie look at the notes in the scale and the interval to root, and working out what the chords would be, eg a note that has Min3 interval to root would be a minor chord].

Instead, I'm going to show you a great webpage that shows you all the chords in a whatever key you choose:
http://randscullard.com/CircleOfFifths/
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Guitar:
- Ibanez S670FM w/ JB
- Fender 'Lite Ash' Stratocaster
- Fender '72 Deluxe Telecaster
- Arbiter LP Jr. Doublecut
Amp:
- Laney VC15

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Last edited by Simsimius at Aug 17, 2010,
#3
Basically, chords made up of notes that make up the key. I'm not going to spoonfeed you the information (too much typing for me!). There are links in my sig, you'll be able to find some chord construction lessons within.

You can add notes outside of the key and still be playing in key though. It comes down to how the piece resolves.