If you want to travel from Brussels to another city, you can do it from a bus terminal like Gare du Nord (North station), Gare du Midi (South station) or Rue du Cardinal Mercier. You can reach a big number of cities from Brussels, and around the terminals you will find everything you need for your trip like ATMs, toilets, luggage storage, etc.
The three main train stations in Brussels are Brussels Nord (Gare du Nord/Noordstation), Brussels Centrale (Gare Centrale/Brussel Centraal) and Brussels Midi (Gare du Midi/Zuidstation). Brussels Nord has no metro connection, but you can take the bus or tram from there to a metro station or the city centre. Brussels Centrale and Brussels Midi are the main departure points in Brussels and the most frequented by tourists. Note: If you want to travel by train, it will come in handy to know both names of the stations (Dutch and French).