Every self-employed person, director, business manager (except for an unpaid mandate), working partner, company, etc. who is not bound by a contract of employment must join a social insurance fund.
This guarantees having certain social security rights and obligations such as family benefits, sickness and disability benefits, maternity insurance, your pension and bankruptcy insurance.
Information regarding options of social insurance funds in Belgium, the right time to do it and details on the contribution can be found here.
Also note that as a self-employed person, you are required to join a health insurance fund (FR) when you join a social insurance fund.
Affiliating to a social insurance fund and a health insurance fund can be done on your own. Accredited business counters also offer these services for a fee.