I would suggest that you take the October ACT and the December SAT. Leave a bit of studying time between the two tests because the ACTs are not like the SATs. The ACTs test is composed of four sections: English, Mathematics, Reading, and Science respectively. And although they don't take points off for an incorrect answer, one question is the difference between a 34 or a 36 on the Science portion.
For the ACTs I would recommend The Real ACT Prep Guide (or Red Book as it is often called). It's available for purchase when you are registering for the ACTs and the practice tests inside are all taken from past ACTs. Other people absolutely swear by Barron's or Princeton Review, but I found that nothing beats the practice tests in the Red Book. Study during the week, take the book to school and just flip through it during lunch or free class periods. Then, take a timed practice test every weekend so that you can get acquainted with three or four hours of testing. Really crank up your studying the week before your test, and take a timed practice test everyday. But don't stress yourself out, give yourself the day off the day before the test and just relax. Hopefully you'll be able to get the score you want!
As for the SATs, the Blue book aka the Official SAT study guide, a few Barron or Princeton Review practice books, and a big box of vocab cards. The studying techniques are sort of the same as the ACT ones, but with a few variations. First, you should probably build your vocabulary, like by a LOT. Reading usually helps, and I'm talking about books like Wuthering Heights, Crime and Punishment, The Grapes of Wrath, Classics! Modern classics are fine as long as they are challenging. Set a goal for yourself like learning 50 new vocabulary words a week and start from there. The rest is really just practice, practice, and more practice. There really isn't much more to it.
Finally as a general rule, focus on your weak areas. If you want to score higher on the math portion on the SATs, buy a SAT Mathematics prep book. If you're a bit fuzzy on grammar, invest in a Grammar Usage and Mechanics book. Well, you get the idea, invest time and money into your preparations!