Speed is the biggest issue for me, but:
Reading comprehension might be difficult if you haven't read much in your life.
Math is pretty basic, just depends on where you're at. Get through basic trig first at least. Do all of the easy problems first.
English is, well... Just make sure you know basic grammar rules, and try to think about published stuff you've read if you get stuck (as in try to remember an example to help you out).
Science - look at the answers FIRST, then find relevant info in the question. Ignore the fluff.
If all else fails and you're running out of time - GIVE IT YOUR BEST GUESS. Leave no question unanswered.
Also, do practice tests. They help.
