Before you leave to go racing in Belgium, make sure you are at a high level of fitness. Racing in Belgium is very competitive and unless you do well at the regional level in the US, you will have a very hard time staying in the races in Flanders. It is probably best to try to peak around the time you go, especially the first time.
Don't set your expectations too high. Most foreign riders don't do so well on their first trip. Several who attend the USA cycling camp for nieuwelingen don't even manage to finish one race without a mishap or without getting pulled. And these are pre-selected riders, so don't let early poor results worry you too much.
Racing in Belgium takes some getting used to. For young people it is best to try out European racing at the earliest possible age, which is 15 years (racing) age or 1st year nieuwelingen. For anyone younger than 17 it is probably best not to go alone. Although it can be done it is better not to, in case something happens and you need medical assistance. At that time, it is better to have an adult around.
Make sure your bike handling and pack riding skills are well honed. You can sharpen your skills by mountain biking and mountain biking is a good way to prepare for Belgium in more ways than one. I mentioned this before in this post.
Expect to ride in aggressive large packs, even in local kermiskoersen and expect the race to be very fast, especially in the beginning. When you go to Belgium, try to arrive several days before your first race so your jet lag clears somewhat. Ride around to become familiar with the roads, the width of the roads, and the pavement types. Go out to where your first race will be held and see if you can ride the course. Definitely arrive early on race day and ride the course. Most are relatively short loops so there is time to explore. Pay attention to the many obstacles that you will have to maneuver around in the race.
Try to ride near the front of the pack as much as you can. Don't overdo it though and don't go pulling all the time. Belgian riders will gladly let you pull and do all the work. But remember, it is the finish that matters, and the key reason for riding near the front is to stay out of trouble, not to show off. To stay near the front, you have to try to start in front and that means arriving early and holding on to your spot.
When you stay in Belgium for a whole season, and especially when you are an U23 rider, make sure not to race too much. Even younger riders should watch their calendar, although the restrictions that are in place for younger age groups do help somewhat. Even so, most riders overreach and burn out. Racing in Belgium is not like racing in the US. If you race several times a week, chances are very high that you will overreach and your performance will suffer.