If you need to firm up a floor, install sheets of plywood. If the new flooring will be too high compared to nearby floors -- most people find a height difference of 3/4 inch to be a tripping hazard -- you may need to remove old flooring before installing the new.
Sweep the floor well, set any popped nails, and remove the baseboard shoes or moldings. If the molding is in good shape, write numbers on the backs so you can remember where the pieces go. Otherwise plan on installing new moldings.
Level any bad dips by pulling up the old flooring, nailing shims to the joists, and renailing the old boards. Staple one or two layers of roofing felt (tar paper) onto the subfloor. This will help prevent squeaks.
Use spacers to create a 3/8-inch gap between the flooring and walls or baseboards. This is important; boards that are installed tight against the wall may buckle. Shoe molding will cover the gap when you're done.