Talk to a recruiter, they'll let you know where the testing facility is. Usually MEPS but if you're still in High School there's a really good chance they offer the test every week/month.
Some students at my old school took the ASVAB on campus, so I'm assuming they do this for a lot of High Schools.
i christmas tree'd the math part and still got a 80Algebra, geometry, basic grammar, simple physics, some automotive questions (Which do not count towards your overall score only certain line scores), and some civics questions. Not too in depth and overall it's really easy, I scored a 95 a month after school was out after junior year (so the thinking part of my brain had been shut off for a while) and if I get to take it again I'm sure I will do the same. I do know though that the grading system was changed so perhaps I won't score as high, all I need is a 93 and I can do whatever I want in the Navy.