EU nationals in Belgium can work without any special permission. They do not need a work permit for employees. However, as an EU national you will need to fulfill the same administrative obligations as do Belgians.
An employee is a person that works under the authority of a formal employer, has a labor contract or an official status as a civil servant. In both cases you will need to pay taxes and social security contributions. Generally speaking, in term of social security, the status of employee will give you a better protection than being a self-employed. Employees will mandatory need to register with a health insurance.
More information on your obligations can be found on the website of the Federal Public Service for Employment, Labour and Social Dialogue.
The following page on the Brussels Region website offers a range of useful information on employment in Brussels.
In Belgium there is a big difference between the gross and the net salary! When you negotiate your salary during your job’s interview it will always be in gross: To calculate wages from gross to net, or vice-versa you can use one of the following applications:
The net salary is the amount that you keep after deduction of income tax, the NOSS contribution, and other permitted deductions.
Labour regulations being a complex legal matter, it might in some cases interesting to discuss your individual case with some government bodies.
More info on Belgium.be