Get it to roll straight by pushing it. You don't have to roll it on an incline, just turn the radio on, and with the controls at neutral, give it a push across some flat and level concrete. If it pulls, then correct that until it rolls straight. Correct it mechanically, not with trim, as this will mess up your rudder trim as well. Then, learn to correct for engine torque by applying opposite rudder. You might have a nose gear that's too sensitive (I'm assuming it's a trike setup). Make sure your nose gear is connected to the innermost hole at the servo, and the outtermost hole at the wheel. This will desensitize the nose gear so that over correction isn't as much of an issue.