Missileman is right; check the belt and the direction of it and the tail-rotor rotation. Second is that your gyro may be out of gain adjustment or bad, to check that disable the main motor by removing its wires from the speed controller (remember the order of the wires) turn the tx on and connect the battery, set the throttle at half or quarter settings, then lift the heli and sway the tail to left and right, in the mean time look at the pitch on the tail rotor and see if it is responding to the opposite direction, if not check the gyro and set it up per the manual instruction.

Good Luck, Raz