There are trim switches by each stick on your transmitter, one for the elevator, ailerons, and rudder, as well as the throttle. If, for instance, your plane has a tendency to pitch down in flight, you'll want to add a few "clicks" of up elevator trim (pull the elevator trim switch towards you). The same process can be repeated for yaw and roll as well.
As an aside, most people trim their airplanes in the air, but if you don't feel comfortable taking your hand off of the stick in flight, you can do adjust the trim on the ground - just be careful to do it in small steps.