Depends what boat it is. If it's a single scull, or a double scull (double is 2 people, 2 blades each), then it is down to pressure on the blades (if you feel like you're going off line a little bit, increase the pressure on one side and push slightly harder with one foot).
If it's a quad (4 people, 2 blades each) or a coxless four or pair, then one member of the crew will have a steering foot (your foot goes into a normal trainer that is on a steering plate) that moves that is attached to rudder wires that move along the boat to attach to the rudder. The foot is moved in the direction you want to go to. The rudder is effectively a brake that steers, so all use of steering is kept to a minimum.Who actually steers the boat is a matter for the crew to decide, as this system can be set up anywhere in the boat.