Check the Park Tool web site. They have freewheel removal tools for a variety of makes.

One key consideration is whether your old freewheel is worn out and headed for the scrap barrel, or whether you want to use it again. If it is no more than junk, the tips of a thin nose plier can be used as a spanner wrench to remove the retaining ring. You might need to clamp the pliers to a piece of wood for leverage. The threads are left-hand threads. Once the retainer ring is removed, the cluster and the bearing balls come away leaving the core. A pipe wrench will remove it.