I would buy an SAT prep book(they're around $20) and take the practice test under realistic conditions. They often include computerized practice exercises, either with a computer or online. Once you have taken a couple practice tests (under timed, realistic conditions), see whether you are happy with your performance and move on from there.
Also, some do better on the ACT test, so you might consider taking that in addition. Many colleges will accept the higher of the two scores.
As for "how important is my score", that all depends on the college. You'll need to research them individually.