No drugs, alcohol or the like beforehand, either. Have a light breakfast. Standardized testing is not the most exciting thing in the world, needless to say, and being sleepy from a heavy breakfast won't help you. Bring along a light snack to eat during the test itself. Bring a jacket.
Most schools offer free after-school test prep sessions.
I highly recommend you attend. The guidance office should have all the needed information. Practice logic games, play a little chess or one of the other mind games to get yourself used to the type of reasoning you will be expected to produce. Buy a couple of the test guides, they run about fifteen dollars. These books have many sample problems and tests that will once again, get you used to what you will be going through.