Also realize that Brussel-Centraal is very busy with about 145,000 travelers a day and trains leaving every minute.
Signage will be in both Dutch and French, so be aware of the actual name of the city you are going to.
Trains don't wait and it's not necessarily easy to find someone to help you. Also, be aware of pickpockets at the airport as well as people offering to "assist" you with your luggage.

Look on the departure board in the main hall for the appropriate platform and be ready to use stairs... crowded stairs