Generally, you'll need a specialized tool if you want to remove the freewheel without destroying it. I think with single speed freewheels, they tend to be four pronged removers. Be sure to buy the one specifically for your make of freewheel. It seems every manufacturer tends to use their own scheme.
There are some freewheels that are not removable by any tool. I don't know why they do this, but they exist, and tend to be low end models. The only way to remove them is by destroying them in the process. You'll have to unscrew the outside ball race, which usually is reverse threaded. At that point, a million little ball bearings will fall out and scatter around the floor. The sprocket/pawls are then removed. Then you can use a large pipe wrench to unscrew the rest of the assembly from the hub. I suppose if you are really careful, you can save the freewheel. Keep in mind, ANY freewheel can be removed destructively. If you have some old 70s wheel with an unwanted obscure freewheel, the wheel can be be salvaged
