Remove the small timing hole plug on the left side of the engine, so you can see the flywheel. Turn the flywheel until the "T" mark is aligned with the notch in the side of the timing hole. You will be on TDC. You can also remove the sparkplug and watch the piston. When the piston comes all the way to the top, it will be on TDC.
If you are timing the ATV, make sure you put the cam in with the lobes facing down. If you put it in with the lobes facing up, you will be 180 degrees out of time.