It spins no matter what I do.

I am assuming you are flying a model that we sell. (See the side note.)

Check these items in the order they are shown.

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Check to be sure the tail rotor is rotating in the right direction

Looking down on the helicopter, turn the blades clockwise

Regardless of which side the tail rotor is on, the tail rotor blade that is closest to the front of the model should be going up and the leading edge (fat edge) of the blade should be leading. If not, you might have your belt reversed or your blades/blade grips reversed. Check your manual to see if the control ball should be leading or trailing. Fix it so both conditions are true and your tail conforms with the manual. (see sidebar)

But my main blades normally turn counter-clockwise.

You probably have a Vario or some other European brand of helicopter. Regardless, the leading edge of the tail blade closest to the front should go up when you turn the head in the direction it will turn when flying.

A model can be made to fly even though it isn't built according to the manual. However it is usually best to conform with the plans.

Be sure the tail rotor servo isn't reversed

When you move the rudder stick on your transmitter to the right, the servo should always pull the control rod that connects to the tail rotor bellcrank forward and the leading edge of the tail rotor blades should point left. If not, reverse the rudder channel on your transmitter and recheck.

Why?

If there wasn't a tail rotor and the model was under power, it would spin to the left all by itself (physics).

The tail rotor's purpose is to control the spinning (yaw). Pretend the tail rotor control rod is a string instead of a rod. If you want to spin to the right, the only way it would happen is if you pulled on the string. Pushing on it sure wouldn't get you anywhere! Remember, the model wants to spin left most of the time anyway. So to yaw to the left is all we have to so is relax tension on the string and it will go left.

Check the gyro direction

Turn on your transmitter, then the receiver. Don't move the model for about 10 seconds. Pick the model up with your left hand holding the head and your right hand around the tail boom so you can feel which way the control rod moves. Spin the model so the nose goes left. While it is turning, you should feel the control rod go forward. If you feel it go to the rear, change the direction of the gyro (see side note). If you don't feel it move at all, go back to the trouble shooting "I can't change the gain".

This step is easier to do before you install the gyro. Just rotate the sensor left (counter-clockwise) and check for the same conditions above.

Most gyros have a little switch that lets you change the direction. It will usually be marked "DIR" with choices for "REV" and "NORMAL". If yours doesn't have a switch, you can change the direction by mounting the gyro upside down.