If you haven't already, turn off the autopilot thingie.

Beyond that, sounds like a rudder trim problem. On many trainers, it's hard to tell if it's the rudder or aileron causing a plane to turn. I'd center the ailerons, then see if you can stop the turning with either just the rudder or with both rudder or ailerons.

Also, double check the aileron linkages. I once took off with one aileron in the wrong hole on the control arm. The plane flew very funny, mostly rolled one way much faster than the other.