SAT exam dates are offered in September, October, November, December, February, April and May. If you want to sit the SAT then you will need to register.
You can do this online, but make sure you register in time and that you don't turn up to do it on the wrong date as you'll have to pay the registration fee all over again.
And speaking of fees, the SAT costs are as follows: Reasoning Test are $52.50; the SAT Subject Tests are $26 (base price, with additional expenses added depending on which type of test you take).
To register online, go to
http://www.collegeboard.com.