bike gear questions

Hi Everyone:
I'm a brand newbie. I am really enjoying this website and learning from other people's posts.
My interest in Triathlon stems from boredom with running and way too much time spend sitting on the couch waiting for nagging injuries and realizing that I could bike and swim instead.
So I signed up for swim lessons last month, that's been great. I had no idea what improvement I would get from a handful of lessons.
A few weeks ago I bought a bike. Having a bicycle is like participating in two activities, cycling and shopping for all of the gear I need for my bicycle. I'm a short guy, 5'5, my bike is a trek pilot 5.2, last year's model, big savings there, and it's 50cm, which is their smallest men's bike. For a short guy I have an even shorter inseam it seems. So with the bike being that small when I went to install the water bottle cages I found out that the one that the one cage is too close to the top tube to really be used. After putting the cage on you can't put a water bottle in. I noticed that there are cages you can put on the back of seats, but the seat post is carbon and I don't think I'm supposed to put something clamped around it. Also, if the water bottle cage goes there then where should I put the necessary supplies for fixing a flat?
By the way, that's my other question, what are the essentials to take out with you on a long ride? I just got the bike and have only done a couple of 25 mile rides and have not even gone to buy a spare tube or anything like that. I know I'm dumb and lucky, but I don't want to chance it again. I'd like to hear what other people consider to be essential gear when out cycling.
Any tri folks in the SF East bay? I'd love to find a coach, mentor, trainer partner, or all of the above.
Thank you,
Chris