I remember taking both the ACT and the SAT, you are going through a tough time period, but I'm sure you will do well.
You obviously want to go to college hence you are going to be taking the standardized test. As far as a good score is concerned, due to the competitiveness of students in the United States, a score of an 1800 is base minimum, TYPICALLY. That doesn't mean if you get below an 1800 you won't go to college, but you will have to make yourself stand out whether it be through activities or your essays.
I remember I got a 2170 and I went to CAL, graduating this year...GO Bears! I also took the ACT and got a 31 (out of a 36)
The ACT and SAT are quite different. The ACT is much more geared towards students who want to go in to Engineering, science, math, physics and more tech fields whereas the SAT is a test that has been around to gage the normal students transition into college.
The reason you even take the tests is so that the colleges can know how you will supposedly do in college. The score shows the kinds of grades you will be getting in college. Is that actually true? Not really, my cousin got a 1620 on her SAT went to UC Davis and currently has a 3.7 gpa...
Before taking the SAT and ACT know that the ACT requires you to take a higher level of math because it tests you up to Algebra II and Trig. The SAT doesn't usually go beyond geometry.
The science section of the ACT doesn't require you to know chemistry per-say, but you should know the first 20 elements of the Periodic Table of Elements, and you need to know how to read and gage charts and conversion tables and graphs and such. You should also know simple scientific math formulas like Density=Mass /Volume, Work=Force X Distance. ACT science is more physical science than conceptual.
This probably seems like a lot to take in, but know that you will do great as long as you keep taking practice tests and keep yourself prepared by reading current events and going to the college board website for the word of the day (they really do help!)
Depending on what you want to do, Science/Math/Tech field, you should take the ACT, if you are on the West Coast, they will take the ACT.
If you want to go into an "Artsy" field or Social Sciences, Liberal Arts, then you should take the SAT. Another thing you can do is take the SAT and then take the SAT II in math and science.
Hope that helps, and good luck!