(1) look for any pipe or wire penetrations through the floor and seal them up with caulk or Great Stuff. Large holes such as you might find around the tub drain can be sealed with a piece of thin wood or foamboard.
(2) Insulate the floor with at least R19 fiberglass batts. Make sure the batts are fully in contact with the sub-floor wood otherwise the insulation has little actual value.
(3) Air seal and insulate the sill. That is the place where the wood frame of the home rests directly on top of the concrete foundation.