I scored lower on the PSAT but higher on the ACT, so don't count yourself out. Is this the first ACT test you're taking? I took it three times during high school and got one point more on each test. If you get the chance, definitely take it more than once.
If this is your first time taking the test, be prepared for the science section; it has less to do with true science knowledge and more with how well you can read graphs and understand them, as well as the information they're throwing at you.
For the math section, I would review trigonometry/geometry and make sure you have a good understanding of Algebra; there will probably be problems on rates (like rate of interest, etc.). Remember that, like in the science section, half the battle is just understanding the problem. Look over the problem, figure out what is given and what you're trying to find, then start solving.
If I think of anything else I'll try to post it here; otherwise, good luck, I'm sure you'll do great!