to make it simple: pick one chord.... say Am. pick a second chord...say Gm. pick a third chord....say Fm. Put them together and what have you got? Nothing! Basically, you have to trust your ear. pick the chords that sound right to you. pick the progressions that sound right. record it and see how it sounds. i dont need to tell you that things sound different when you record them or that what sounds good to you may sound like crap to someone else.
Start out as simple as possible and add on as you go along. Usually the first chord you play is the key of the song. but again, play what sounds right to you.