Belgium has hundreds of bike clubs that are members of the Wielerbond Vlaanderen. You can find a complete list on the website. You will notice that clubs are listed as clubs met renners (clubs with racers), wielertoeristenclubs (cycling tourist clubs) and organizerende clubs (clubs that organize events). Check one of these options and click the zoek (search) button to see a list of clubs in that category. The listing shows the club's location and the contact information.
You can select alle provincies (all provinces) or choose your favorite province. You can also sort clubs (sortering) op postnummer (by zip code), op gemeente (by town), or op stamnummer (literally by tribe number).
Flemish provinces are West-Vlaanderen, by the North Sea and around Brugge, in the upper left corner; Oost-Vlaanderen, home of de Vlaamse Ardennen (Flemish Ardennes), Oudenaarde and Gent, to the east; Antwerpen, the Northern most province around the port city of Antwerpen. Limburg in the upper right hand corner around Hasselt, and Vlaams Brabant, the Flemish part of the central province around Brussel.
Zip codes are four digit codes that start with 8 in West Vlaanderen, 9 in Oost Vlaanderen, 2 in Antwerpen, 3 in Limburg, and 1 in Brabant. The provincial capital has three zeros so Brugge is 8000, Gent is 9000, Antwerpen is 2000, Hasselt is 3000 and Brussel is 1000.
Also note that a club with racers has separate teams for every age group they serve. Most clubs that support youth racing do so from miniemen up to the junioren level. They will usually have an aspiranten team, a nieuwelingen team and a junioren team. Some of the best nieuwelingen and junioren team clubs do not have a beloften (U23) team and so riders are forced to switch when they reach 19. In most races beloften race with Elites z/c (amateur riders).
It is possible to join a team for a race or a set of races if you contact them beforehand and they have slots available. Usually a team will have many more riders than they can field in an interclub event and internal competition for these events is fierce. However that does not mean you can't get a slot. Due to age restrictions, younger riders will often have to give up a spot in order to remain eligible for other events. Sometimes riders will give up prime slots in stage races (where they are restricted to just a few events per year) in order to race in other countries with the Belgian national team.
For youths it is generally easiest to join a team for a junioren event. Here is where you often find the optimal conditions in terms of age restrictions, number of available team riders, and number of available events. It is also fairly easy to join a team for a Elite z/c event (i.e. amateurs in the open category). Furthermore, by age 17 most solid riders have enough of a resume to convince team managers that they will live up to expectations. Without a resume that shows at least some racing in Belgium, convincing a manager may be tough to do.
The various cycling houses have affiliations with local teams and if you show good race results while staying there, you may get a slot for a team-only event. But it is not required that you stay at one of these in order to make a team. Getting yourself noticed during a race and talking to potential managers and riders afterwards is often enough to do the trick. Nearly all big races from the nieuwelingen level on have talent scouts in the crowd. At a nieuwelingen stage race last summer I noticed Omega Pharma Lotto, Rabobank, and RadioShack along the course.
Going on training rides -almost always welcomed- with a team is another good way to become visible. Many teams are attached to a local bike store and hanging out in the store (especially in the back where the mechanics are) will get you connected to the right people pretty quickly. Although nearly everyone speaks English, it is definitely helpful if you learn some words in Flemish. And the best place to learn relevant words is to go on rides with people.