I borrowed this from another post by Icvette. He is an expert.
To determine Top Dead Center, you need to remove the number one spark plug, use a spark plug hole piston stop and turn the motor until it stops against the piston stop. At this point you need to add some form of stationary pointer to your engine near the harmonic dampener that will be visible with a timing light from above the motor. Doesn't matter where the pointer goes as long as it is sturdy and will not move on you!
Once the pointer is installed, make a mark on the dampener where it is pointing too. Now, turn the engine over the opposite direction until it again hits the piston stop. Again make another mark on the dampener where the pointer is pointing to.
Now, the center position between those two marks you just made is true TDC for the #1 cylinder and is what everything in your ECM is calculated off of including ignition timing, fuel injector timing etc etc.