When you get a phone in Belgium you have two options: a prepaid card or a contract. You will need to contact a provider in order to set up your card or contract. Most providers have stores you can go to where you can set up the contract, some will allow you to set it up online.
For more information on providers: https://www.mesfournisseurs.be/telephonie/operateurs
Some providers will offer you package deals if you take more than one service (phone, internet, TV)