I want to go from Gare du nord to terminal 2 of CDG airport. Which station of gare du nord shall I go and which train shall I take

Inside Gare du Nord, look for signs for RER B - Mitry Claye Aeroport CDG 2 TGV (see the linked article for photos of these signs).  This the area you need (RER train area) and the train + direction you need (RER B  → Mitry Claye Aeroport CDG 2 TGV).
 
The terminus station is your stop: CDG 2 TGV, so ride the RER B until the end.  The RER B also stops at CDG 1 train station for CDG Terminals 1 & 3 before continuing onto CDG 2 train station so don't get off early.  The terminus RER B station is located inside CDG Airport Terminal 2.