Answer:
Generally in a single scull (where there is just one person) the Sculler (the person sculling) will pull harder on one blade thus turning the boat in the opposite direction.
This is achieved by pushing harder with one leg.
In bigger boats, they often have a rudder and a stearing mechinism to turn the boat operated by someone in the boat called the coxswain (who does not row) or often in smaller boats such as fours and pairs by a mechinism on the scullers footplate that turns the rudder.