assuming your dropouts are viable, just get a new set of wheels that are specifically for SS/FG riding. They aren't that expensive - $300 tops maybe. Just get the set that matches your rear spacing. buy a $20 freewheel, screw it on. ride.
start there. from there you can fuss with a new crank and/or bottom bracket spindle length if you want to make it real nice - otherwise most likely 1 of your chainrings on your multi-speed crank will line up with the back pretty well.