How do I interpret the requirements to join a specific section of the Marines?

I would really like to join the marines. And the more and more I research about you guys, (and gals.) I think I've came to a conclusion, I would love to become an MP (Military Police), and when I look it up, there is so much to do to get in. You have to score a certain amount of points on the ASVAB, and I've only took the ASVAB once. And I scored a 30. It stated that you need a 32, and there is no lee way. I'm currently a junior in high school, and next year I can take the ASVAB again. But the only thing that freaks me out. Is that if I dont make a 32, then I wont be able to join the Marines. And then there is another test, I think it was a GT test. And you would have to score a 100 on it. And I couldn't figure out what GT was. Eventually found out what it stood for. And correct me if Im wrong. General Technicians test. I dont know if I will take that at the base, or if I will take that at school. And if I have to ask for it. Or what. When it comes to this stuff, I dont know anything. So if anyone can help, that will be most helpful?