The 215 multiple choice questions tests you on English, reading, mathematics and science reasoning. Also, if your intended major is undecided, there's a little questionnaire that points you to a broad career category that you might have the most interest and success in. The ACT and SAT are two very different tests, providing two different measures of your abilities. The ACT has a broader range of skill areas. The SAT measures verbal and math skills, while the ACT examines your skills in English, math, reading and science reasoning, covering a wide range of skill areas.
Most schools accept either test, but check with the admissions officer to see what's preferred.
The best way to prepare is to take challenging classes and understand what you've learned. But it's also a good idea to get familiar with the test. You have many opportunities to do that. One way is by using ACTive Prep. This CD-ROM guides you through a test prep program using real timed ACT tests.