No one likes taking the SAT's. It is a three hour test that supposedly measures your verbal, math and reasoning skills. However, many people debate whether or not the SAT really is an accurage measure of your intelligence or skills. Still, an entrance exam (either the ACT or the SAT) is absolutely necessary for 99% of the colleges you will apply to. Some of the more progressive and liberal institutions weigh your high school experiences and your college essay more highly than the aptitude tests, but most institutions still require it.
The SAT should be taken your Junior year of high school. At some point in the six months before you take the SAT, you should take the PSAT, which is the exact same exam as the SAT, and it gives you a grade just as you would receive for the SAT, but the grade doesn't go on your record or count for anything. The PSAT is just an indication of how you will fare on the SAT. It may guide you to decide how much SAT preparation that you will need. The Princeton Review puts out a wonderful book to help you study for the SAT. It includes study tips and practice problems, as well as likely SAT vocabulary and math problems. Also, you can complete practice SAT problems on www.collegeboard.com. This site is also where you can register for the SAT.
An alternative to the SAT is the ACT , which many schools accept in lieu of the SAT, or in addition to the SAT. The ACT includes a science section and is constructed a bit differently than the SAT. It may be advantageous to take both tests and see which you fare better on.
These entrance exams are not worth obsessing over. You should do as best as you can to prepare fully for the test so nothing comes as a surprise. Still, the SAT is hard to study for because it covers all of the information that you should have learned in high school. If you are really nervous about the SAT, you may want to enroll in an SAT prep course, such as Kaplan. Students are known to earn better scores after taking such a class, but the courses are usually pricey. If you are applying to extremely competitive colleges and you are nervous about your scores, a prep course may be a sound investement.