Paris to Brussels

There are 3 stations in central Brussels:

• Bruxelles-Midi / Brussel-Zuid (English: Brussels South)
• Bruxelles-Central / Brussel-Centraal (English: Brussels Central)
• Bruxelles-Nord / Brussel-Noord (English: Brussels North)

High speed trains from Paris (Thalys and TGV) stop at Brussels South. You can either get out there and walk or take a tram or métro to wherever you want to get to in central Brussels, or take a local train to one of the other stops (Brussels Central or North)

That's quite a walk, and the tram doesn't really go easily where most tourists want to go.

For the Grand Place, take any of the very frequent local trains the one stop to Brussel Centraal and go upstairs, then go outside and walk down hill for about 3 minutes.

Bourse stop on the tram routes 3/4 from Midi is very close to Grand Place.

Agreeing with Nigel, to keep it simple just change platforms in Brussel-Zuid (where the high speed trains stop) and take a local train direction Antwerpen/Anvers, you don´t need to buy an extra ticket for the few minutes ride to Brussel-Centraal if you have the ticket Paris-Brussels still with you (to be sure ask the ticket inspector). For the tram you need to buy an extra ticket and you need to walk actually outside the railwaystation, so you won´t gain much time.