Yes, AoPS is the place to be. There are many courses, each with textbooks. But for just general study, start with The Art of Problem Solving: Volume 1. It’s well-known in math circles to the point that you only need to refer to “Vol 1” and everyone knows what you mean.

There are also decades of old AMC tests with solutions/explanations. The courses are great - I’d recommend the beginning Number Theory and Counting courses, but for test prep, Vol 1 and old tests are best.

There is a Vol 2 to cover advanced material for AMC12 and AIME.