For your purposes, a cog/lockring would go on the fixed side, and a freewheel would go on the free side. While you can thread a cog onto either side of a track hub, you can't thread a lockring onto the free side, which means as soon as you backpedal the cog will unscrew itself and your bike will explode.

The fixed side of a track hub has two sets of threads. There is a set of right-hand threads that the cog screws into, and then a set of left-hand (reverse) threads that is used to screw a lockring against the cog. As you resist against the pedals when riding fixed, a track cog has a tendency to unscrew itself. The lockring is there to prevent that from happening.