I've been practicing the SAT, but after taking the January SAT I feel that my SAT score isn't sufficient for the colleges that i'm aiming for so i've decided to give ACT a try. I've been averaging around 2100 on the SAT, but couldn't seem to get up to a 2200. I will be aiming for at least a 33 on the ACT. Is there a big transition between studying for the SAT and applying concepts on the ACT? What can I do to prepare myself for the April ACT?
The ACT is challenging for many students because it involves more time pressure than the SAT, so I would recommend that you take plenty of practice tests under realistic (timed) testing conditions to get used to the faster pace.
You should also practice ACT math problems that incorporate concepts that are not tested on the SAT. The ACT includes some questions on trigonometry and conic sections. These wouldn't have shown up on the SAT Math section on the version of the test that you took (although now that the SAT has been revamped, in the future there will be a few trig questions).
Another major difference between the SAT and the ACT is the inclusion of the Science section on the ACT. Don't be too worried about this; you don't need to know anything beyond basic Science to do well. Just practice the Science section as many times as it takes for you get used to the format. ACT Science is essentially fast-paced logical reasoning in a scientific context. Beyond learning to look past extraneous information, you won't need to do any extra outside review of the material.
There's not as big of a difference content-wise between the two tests as some people seem to think, but it's definitely helpful to do some extra practice before diving straight into an unfamiliar test.