The ACT, or American College Testing, is basically the same thing as the SAT (Scholastic Aptitude Test), only easier. The multiple-choice portions on the ACT include English, Math, Reading, and Science, as well as the optional Writing portion. It costs $47 with the writing portion, or $35 without the writing portion. The SAT includes Critical Reading, Math, and Writing. It costs $45.
If you're looking for an opinion on which test you should take, it all depends on you. The ACT is definitely easier than the SAT, but it includes Science, whereas the SAT doesn't. If you feel strong enough in Science, go for the ACT.
To prep for either test, it's best to go to the website where you registered for the test. If you registered for the SAT, go to collegeboard.com. They offer an option to sign up to receive one SAT "Question of the Day". They also offer courses you can pay to take online to help prep you for the test. Not interested in paying? They have a free SAT practice test you can take, and it explains the answers to you. Or, you can do it the old-fashioned way and head over to your public library and check out an SAT prep book, but you can't write in it.
To prep for the ACT, go to actstudent.org/testprep/index.html. There you have basically the same options the College Board website.
Hope this helped you:)