Your first tri! That's exciting. I did my first last September. Here's a link to my blog post about it.
http://tris3kidsandlife.blogspot.com...ri-report.html

A few posts before that one talk about my training. I chose a sprint tri for my first race, it seemed the least intimidating.

Ditto on what Alpine Rabbit said about bricks. They're really hard, but if you do them, the run will be easier during the race.

Also, check out http://www.beginnertriathlete.com/
It's a great source for information and support.

As for why I decided to do a tri, I think it was because I had turned 40, and was looking for a way to challenge myself. It's something I wanted to be able to say that I did, but now I'm pretty hooked.

Good luck and have fun!