with your budget, it's going to be tough. my understanding is that central cal has a robust racing scene. at 18 y.o. you're a jr. for a few months. lots of racing clubs have jr. programs that can assist folks such as yourself in finding inexpensive equipment to get started in the sport. check out www.usacycling.org for clubs in your area, get the contact info and start calling them to discuss your situation and see if any have a program that can get you going.

good luck.