Zoey,
Your situation reminds me of where I was two years ago. First of all, congratulations-you've already taken great strides in that you know what you want! I hope that I can offer some useful advice; I realize that this has become the cliche of all cliches; but it does actually get better, especially if you have a well-thought plan.
Nobody can tell you whether or not to transition. I know that I literally couldn't wait another day after I graduated from high school, and I immediately went full time the day I left home. If you feel the same way, than certainly, it's worth considering. Whether risking homelessness is worthwhile is a decision only you can make. Given your family situation, it might help to try and test the waters. Talk vaguely about "a friend", or present in a slightly more feminine way, and see how they react. I took the risk without knowing how my parents would respond, and though my parents have actually come around this past year, it was very touch-and-go for a while. Anyone who has experienced displacement and homelessness, especially someone who is transgender, can tell you that it's very hard to get back out of it. I've been very fortunate that I never really lost all of my support. Not everyone is so lucky, and it's very difficult to survive on your own.
Therapy is usually expensive, but I cannot even begin to tell you how invaluable it can be, even if this person won't help you acquire hormones. One possibility would be to see someone with your parent's knowledge if they'd be willing to support you in that endeavor, and not to tell them what you're discussing with the therapist. Perhaps this makes me a tad conservative, but I think that everyone should speak with a therapist before transitioning . I don't know southern California at all, but I do know that a number of LGBT health centers in New York (my humble little home) offer low-cost, scaled pay therapists who would be more than happy to help you start hormones. Perhaps a counterpart exists near where you live.
I think you're facing a hard decision: Frankly, if I knew I would have to choose between an education and transition, in retrospect I would probably have chosen education, just because it becomes incredibly difficult to do later in life, even more so than transitioning. I'm very happy I never had to make that choice, and I'm disgusted that anyone ever has too. One possibility would be to try and begin transitioning quietly, presenting as female only part-time, until you can get away from home. I actually have a close friend who did just that during his year at a community college; when he transferred to a 4-year school, he went full-time and is doing quite well. This is really a worst-case scenario, but if you're sure that you have no chance of gaining your family's acceptance, it's always an option. While the prospect of keeping a secret like this scared the hell out of me, it gave me a lot of breathing room while I was still exploring my identity, and I think waiting to come out to everyone might be a worthwhile prospect if it gives you some security.
If I'm reading this correctly, it sounds like you haven't really met many other LGBT people before. When I was just first taking my first steps, I went to an LGBT youth conference (with a change of clothes in my bag, as I was depending on my father for a ride). I met one of my best friends there, and it gave me a chance to just be myself in front of a community of people who were loving, accepting, and just downright fantastic. Plus, I met a drag queen who landed me a job that ultimately gave me the portfolio I needed to get into a top theatre school as a costume designer. But my point is this-if you can, find a community center, or a conference, or just a circle of friends. Take the bus, or hitch a ride...but do whatever you can to find a group like this! The prospect of transitioning became so, so much easier once I made friends who were doing the exact things that I was at the exact same time. It will really make a huge difference.
I hope my late-night ramblings don't seem too incomprehensible. I hope that things turn out well for you, and if you have any other questions, feel free to post here, or PM me-it's almost amazing how much your story sounds like mine (right down to the initial college rejections, I fear!) and I really hope that your story meets a happy ending.
Best of wishes,
Sasha