Throughout your high school career, you'll probably end up taking the following exams:
Advanced Placement (AP)
Some colleges allow you to get college credit for certain AP classes in which you receive a 4 or 5 on the test (depends on the college). Most high schools also give you a higher GPA for getting an A in the AP class, i.e. so you can have a higher than 4.0 GPA.
Most colleges publish ranges (25th - 75th percentile) for their admitted students. While it's not necessary to fall within this range, it's definitely a good idea if you can since it's an objective measure of aptitude, compared with more subjective measures like your extracurriculars or accolades.
It's a safe idea to take these tests atleast once in 11th grade since you want to have a backup score in case you don't do as well as you would have liked in senior year. It's also nice to not have to worry about it so you can focus on applications.
SAT II Subject Tests
Some of the more engineering-based colleges may require 2 SAT II’s (one in math, one in sciences) and some don’t even require any SAT II subject tests at all. In general, it can be a good way to showcase your expertise in the subjects you are passionate about. If the college you are applying requires you to specify/apply to a particular major, then it helps to show that you have high SAT II scores for that subject. If you have the time and expertise, I’d recommend taking one in math, one in science, and one in History/Language just to have all your bases covered for the various requirements.
Again I'd recommend getting these knocked off by 11th grade, if you can.
PSAT/NMSQT (Preliminary SAT/National Merit Scholarship Qualifying Test)
Administered in 11th grade for most high schools, and can be used to receive scholarships towards college