To enter Belgium, you need valid travel documents. For most non-Belgians that is a passport and a visa.
With valid travel documents, you can stay in the country as a visitor for a maximum of three months. If you intend to stay longer, you need to register with the local authorities.
Are you a nationals of the European Union, Iceland, Liechtenstein, Norway or Switzerland?
If so, you can simply register at the town hall of the commune and receive a residence permit (comparable to an identity card).
As a temporary measure, Croatian nationals must obtain a work permit until 30 June 2015.
If not, you need a work permit first
For other nationalities, a residence permit will only be delivered if you have a work permit
Once you have the authorisation to work in Belgium, you have to apply for a residence permit (verblijfsvergunning / permis de séjour) via the local Belgian embassy or consulate. The application must be filed with two passport photos, a health certificate and a certificate of good moral behaviour. The spouse and dependent children under the age of 18, can come and live in Belgium under the same residence permit. More information can be found here.
When arriving in Belgium, you have to register with the local authorities of the town. Please note that this registration triggers the presumption that you become a resident for income tax purposes.