you need to buy a singlespeed freewheel, to thread onto that freewheel hub. more importantly, you'll need to respace and/or redish that wheel so the single freewheel is in the right position relative to the chainring.

respacing it will involve taking off that massive spacer and replacing it with something else. redishing it will involve loosening some spokes and tightening others, to bring the rim back to the center. see bobs garage for help respacing/redishing. good luck.