You won't get far in Belgium without an ID card, for which you'll need an address. Once you've found somewhere to live – even if the address is only temporary – you have to visit your local town hall (gemeentehuis/maison communale) register, and get your ID card.
What will I need?
If you're an EU citizen, you will need a valid passport, three passport-sized photos and proof of financial support (in most cases, a letter from your employer). Non-EU citizens need to provide five photos, a medical certificate signed by a licensed doctor, a certificate of good conduct from your country of origin, an official marriage licence (where applicable) and birth certificates if you have children. Be aware that some communes may want to take your fingerprints.
How much will it cost?
The cost varies from one commune to another, but it is generally no more than EUR 15 (payable in cash only).
How long will it take to get my ID card?
The first time you visit the town hall, you will be given a document confirming your registration and payment but you will not receive your ID card just yet. The police first have to verify that you live at the address you have given. The commune will contact them to confirm that this is the case. If everything is in order, you will be invited to collect a temporary permit that is valid for three months. This will be renewed for a further three months once the commune has verified your proof of employment or self-employment and your social security registration.
You will eventually receive a permanent five-year card. It is advisable to carry it all times. Not only will you be asked to show it when purchasing certain goods, the police can also ask for it at any time. You can be fined if you don't have it with you.