The best place to go for all things SAT is https://collegereadiness.collegeboard.org/sat?navId=gh-sat.
The best way to begin preparing to the SAT is take a sample SAT. These are available free on-line from the SAT at https://collegereadiness.collegeboard.org/sat/practice/full-length-practice-tests.
You can also take a free practice test at Khan Academy (powered by the College Board) that will then provide you with personalized practice: https://www.khanacademy.org/sat.
Also on Khan Academy you can take short practice quizzes that will feed into your study plan to help you practice even more.
To find out what to expect on the SAT visit https://collegereadiness.collegeboard.org/sat/inside-the-test.
The free practice app (available for Apple and Android devices) provides you with a question a day to study as well as an archive of questions you can practice with on the go. https://collegereadiness.collegeboard.org/sat/practice/daily-practice-app.