Taking the SAT in 10th grade isn't worth the money, especially since your already taking the PSAT.
If you take it later, you'll have a better shot at a higher score - 1) SAT scores generally increase every year and 2) you'll learn stuff at school that pertain to the SAT.
The PSAT will give you a feel for the SAT. You can extrapolate an approximate SAT score from your PSAT score. And if you insist on getting a "baseline SAT score,"
buy the Blue Book and take a practice test.
As far as prep goes, do as much prep as you can without hurting your GPA. Allocate some time everyday to SAT prep - e.g. spend 10 minutes on the Question of the Day. http://sat.collegeboard.org/practice...ion-of-the-day
It's never too early to start prepping for the SAT, although taking the SAT in 10th grade though can be a little premature - have you taken Algebra I, II, and Geometry yet?
Good luck in your SAT endeavors