I ditto what KSH and Grog have said (they know their stuff).

If the bike ride is relatively short, and you're not sure if you want to invest money in a pair of tri shorts, you can do the bike ride in a regular pair of shorts (not padded bike shorts, but like running shorts) that you can pull up over your swimsuit in T1. That's what I did for my first tri. It was a sprint (400m swim, 12 mile bike, 2 mile run). I just swam in my swimsuit, pulled on running shorts over my suit and threw on a shirt, then ran from the bike in what I was wearing. If it's not a long distance, your tushy probably won't hurt in regular shorts provided your seat is comfy right now.

I didn't want to invest a lot of money until I was sure I wanted to do this sport very often. It can be a pretty expensive sport if you purchase all the things you think you'll need right up front. Do this first tri, see what you might like to invest in for the next one (which you will do, because it's very addictive), and then go from there.

Best of luck- we expect a report after you complete it!!!