Clean it well with a wet dry vacuum.
How to level floors for hardwood installation.
Make sure you apply a cementitious flooring leveler you can nail through later.
Then using a floor leveling compound or asphalt shingles shim the dips so the floor is level.
If the area is very large use a long straight 2 in 4 in 5 1 cm 10 2 cm board and set your level on top of it.
Pound nails into the subfloor with a hammer and set screws slightly below the surface of the subfloor with a screwdriver.
You will have to make sure your joists can handle the addition of considerably more weight from the plywood subfloor and any floor coverings.
Floors are rarely perfectly level.
On the top side of the floor another fix it idea to lay down new hardwood over the existing floor.
If the floor is uneven lift up the board until it is level so you ll know how much flooring compound you ll need to even it out.
A plywood subfloor will bridge any minor waves in the existing floor and leveling compound would help too.
Hardwood floor installation can be slowed by an uneven subfloor.