American Regions Mathematics League
The American Regions Math League (ARML) is a mathematical problem solving competition for primarily U.S. high school students.
How to participate in ARML
Find out if your area has a team already. Contact ARML for details. If your area does not already have a team, ask ARML how to start one.
See the How to join an ARML team wiki page for more info.
Resources
Books
There are four books of past ARML problems, though some of them are out of print and/or hard to find, but very worth it.