What you got is a long-stay visa so you can indeed travel to other Schengen countries as explained in http://travel.stackexchange.com/questions/19147/can-i-visit-schengen-countries-on-a-type-d-german-visa

As far as Belgium, France, etc. are concerned you have the right to stay there for up to 90 days and in most cases you won't even see any border guards when travelling between these countries and Germany.

But you got this visa for a reason and German border guards will probably ask about your plans when entering Germany. If you got a visa, e.g. for studies, and your first trip is purely for tourism, you might get in trouble with them.