Check out the AMC/AIME/USAMO series on-line. Find out what you can. I would also suggest going to
http://www.artofproblemsolving.com, going to the forum, and asking. There are plenty of resources there for you.
If your school does AMC, you can just go ask to take it. If not, you could probably request that they order the test and submit it. If you do well enough, you can advance to AIME, then USAMO. Now, keep in mind, these are not, by any means, easy tests. They are very difficult, but they are manageable with practice and dedication. You should be able to take the AMC, and I highly suggest you do. It's national from the beginning, by the way. No regional competitions. You just (hopefully) advance to the AIME, and (hopefully) advance to the USAMO, and so forth. Look on that AoPS website and check out their resources.
I don't know much about middle school. TMSCA State is a big (ambiguous word choice) deal, but it's not as big a deal as UIL State. TMSCA State sort of lets you get a feel for the upcoming UIL competitions and the people you'll be facing there. It also encourages underclassmen to do UIL competitions. Last year was my first year (actually, first year my school ever went) there, and I had a pretty good time, being in San Antonio I believe with friends, sizing up the competition, getting more experience in general, and meeting new people. Lots of schools show up, and it's the biggest meet before the actual UIL season starts.
Out of curiosity, have you ever heard of or done MATHCOUNTS? I never did, but it's a good transition into AMC and beyond.