Other Resources
Below are some additional resources for those interested in getting into competition math.
Art of Problem Solving's introduction book series
The Art of Problem Solving website at aops.com also offers many free resources, including math games, videos, and past competitions.
Khan Academy is a great place to strengthen the basics. While it's generally more school-oriented, the geometry is quite relevant. Khan Academy's SAT practice is also great, for both math and English.
Here are some math contests for starters:
The Math Kangaroo contests run at all levels from elementary school to the end of high school. MATHCOUNTS is only for middle schoolers (6-8), and has local, chapter, state, and national divisions. The American Mathematics Contests are the most widespread competitions, and they go all the way up to 12th grade. The AMC8 is a good way to get familiar with the AMC format, as well as contests in general. Also look out for local in-person contests--they tend to have side events and are very fun and a great place to meet other math students!
In the end, everything listed here is just a suggestion and not a rule. There are countless resources out there--YouTube has great videos, and you can find PDFS online compiled by people who are passionate about and experienced in math. The most important thing is to keep enjoying math; and to go at your own pace and do what YOU find interesting. Ultimately, competitions don't really reflect mathematics as an entire field, and are more of a way to foster an interest in math.
https://artofproblemsolving.com/blog/articles/pros-cons-math-competitions