Hi Jenl81,
I was pretty much in the same boat as you and didn't take a prep course. I was really worried before the test but I don't regret it now. I studied over the summer, bought the big Kaplan book, and did all of the practice tests I could find. The Kaplan book has great summaries of all the courses covered (organic, biol, general chem, and physics.) I studied these as if they were my notes.
Definitely try to buy the AMCAS old tests...these are most like the MCAT. Your public and school libraries will probably have practice books. Also, does your school have an education/ tutoring center? Often these have practice MCATs and prep books. Try to do as many practice questions as possible. Even if you feel that some of the questions are not like the MCAT (like the Barron's books), I would go ahead and do them anyways. It's really good practice just to do a variety of questions where you're not really familiar with the format, especially if they cover the same material as the MCAT.
The key, I believe, is knowing the material really well and doing lots of practice questions and tests. The questions are unlike most tests you've probably taken, but with practice, they'll become easier. Also, do timed practice tests to make sure you're able to pace yourself well.
Good luck!
Nick