All non-European nationals (with certain exceptions) who come to work and live in Belgium, have to apply for and obtain a work authorisation before they start working.
For employees, the employer needs to apply for a work permit (for work and stay in Belgium for less than 90 days) or a single (combined work and residence) permit (for work and stay in Belgium for less than 90 days).
Self-employed professionals must apply for a professional card ("Carte professionelle / Beroepskaart"). Under certain conditions, the non-EU national can benefit from a work/single permit exemption. The list of situations where the applicant can request an exemption is available here.
To find out which permit applies to your situation, please consult the webpage of the Ministry of Employment from the Brussels of Capital Region and the webpage of the Ministry of Internal Affairs.