its all about practice man. record mixes and listen back on it, take notes and try again. thats how i got better and better, till i worked up and got residency at a couple of places.
being nervous, imo, its a good thing. means you actually care about DJing and want to do a great job.
i've seen far too many DJ's that have the "so what" or "i'm top shit" attitude, they just sucked... a new guy who just started out that was nervous as balls rocked a crowd and was 100 times better then some of the headliners we had.