I wouldn't recommend purely online tutoring. Test preparation needs to include 3 components: content, critical thinking, and crisis prevention.
you say you "suck" at common knowledge. Well, the good news is that the SAT and the ACT don't depend on a lot of "common knowledge". However, some knowledge of mathematics and grammar is required. The key is to identify what your weaknesses are with regards to the test. Please note that vocab is NOT a huge part of the new SAT. Vocab-based questions are only worth about 180 out of 2400 points! Focus your content studies on relevant math and grammar rules.
This is the most important part of your test prep. After all, the SAT is officially a *reasoning* exam. These are not math, grammar, or reading tests. You need to learn strategies for managing the reading passages, the writing sections, the tougher math questions, etc.
Let's face it, funny things happen on test day. You need to have a few crisis-prevention strategies in your tool kit. What do you do when you have 5 minutes left on your essay? What do you do if a Reading Comp question makes no sense? What happens if you mis-bubble? Spend some time thinking of all the worst-case scenarios, and how you can address them on the test.
Tutoring works when it addresses these 3 components. I've said it before - I'm biased, but I also have inside knowledge of our product and I'm convinced that Kaplan SAT/ACT prep can get you higher scores.