Depending on the year, make, and model, there is four set screws around the outside of the fly wheel.
Those must be backed off before you try to remove it.
If you don't have the Removal tool such as shown below:
There is a couple of different ways you can remove it depending on how tight it is.
Sometimes you can put two bolts in the front of it and then use a lever to loosen it up.
Other times when they are really stuck I have used a set of Vice Grip Locking Chain pliers such as shown below:
Probably some other good ideas as well..