The post directly above has good information on how to go about getting into USSA racing: particularly the last part. If you want to do it, I think the first thing to do is to find out the local race team(s) and get the information about how to join.
Step 1: go to ussa.org, click on the "members" drop-down menu, pick "Membership tools" and then "Club Directory" in the sub-menu. This'll give you a list of local clubs, and what sports they're involved with. You're interested in those which offer "A" (as in "Alpine"), in case that's not utterly obvious.
Step 2: check out the websites of the club(s) that are located where you are. Most should be pretty informative. Some may not be kept as well up-to-date as others, so you may need to do a little e-mailing or phone-calling.
You can also easily find information on how to join the USSA at ussa.org also.
On your initial question: the likelihood of humiliation depends on a few things, including - most especially - your propensity for being humiliated. Since you're already a J1 (by age), it's very likely that the best racers will beat you badly. Then again, they'll probably be a fair number of other people that'll also be getting beat just about as badly. Generally, my observation is that there's usually good camaraderie among people on a team, without regard to their results. Indeed, I'd say the slower racers get more encouragement and pats on the back from teammates than the faster ones. Because it's not really a team sport, exactly, it's no skin off your teammates nose if you don't do that well.