Graduate School Entrance Exams
There are two types of GRE exams: the general test and the subject test. Almost all graduate schools require the general test, but most do not require the subject test. Whether or not you take the subject test is up to you. Be sure you prepare for any exam you are going take. Practicing the exam ahead of time, makes it much easier when you take the real thing.
The following are links to information on the GRE and LSAT (if you are interested in law school).
You should plan on taking the GRE in the spring of your junior year or fall of your senior year.