The PSAT (and any other standardized) test is daunting at first, but with practice it will seem easier. My advice is to actually take the test once through with the real time limits, so you can get a sense of where you are right now. Who know? You might do better than you think! *Make sure to go through the entire thing without any extra aids. The first time, it is always tempting to give up halfway or look up the words, but it's really important to gauge yourself right now.
For the vocab, it might be a good idea to get an SAT vocab book like "Direct Hits". For math, I find the best way is to just do lots of practice problems on old PSAT tests and work at them until I figure them out (that is, after I take the entire practice test).