You should only go a hospital’s accident and emergency department (les urgences/spoed) for an urgent illness or injury. For all other medical needs you should make an appointment with your own doctor or, if out-of-hours, contact the on-call doctor via your surgery.
In a medical emergency, call 100 for an ambulance (ambulance/ziekenwagen). In Brussels you can call 105 for a Red Cross ambulance. Most emergency operators will speak English but talk slowly and clearly in whatever language you use. The ambulance will take the patient to the nearest hospital with an accident and emergency service (not all hospitals have them).
The patient will be charged for the ambulance but if there’s refund, only private insurance will do this (only partly), not state health insurance.
Some medical services and health insurers insist on a doctor’s referral before calling an ambulance unless the situation is life-threatening, so talk to your own doctor about this before you actually need to use the emergency services.