If you set the aileron servo on the same channel you had previously used for rudder on a 3-channel plane you can just merrily go ahead and fly with no difficulty. Most sport flyers just turn with ailerons (plus some elevator). For casual flying you will probably only use the rudder for taxiing. When you start to learn some basic maneuvers you will need rudder - such as for stall turns and spins. Some planes, especially scale types, can not be turned without skidding unless rudder use is coordinated with aileron. But you don't have to worry about that at this point.