2+ on what rocknrollgirl said. You have to just take each part as it comes, although you do have to rein yourself in a bit on the swim and bike to save something for the run.

The swim is my worst part. When I look out over the marked swim course before a race it always looks sooooo long. All I can say for the run is to just do your own race. Don't let someone swimming past you freak you out. Once you've lost your composure on the swim it's hard to get it back. Try to get a rhythm and stay in your own head. Then, suddenly, after what seems like a long time, it's over and you're on your bike.

I agree, too, that the first one should be for experience, only. Worry about you time on the next one. If you stress about having to make a certain time you'll almost definitely blow up.