You may want to put a caulk in the joints before using a liquid leveler. Some can be feathered out to zero, some need to flood and cover the entire area to a minimum depth, so read the specs carefully. Some require a special primer to be installed first to ensure a good bond. A self-leveler over a wooden subfloor may require installation of metal reinforcing lath, so maximum thickness can be an issue.
You need to use a long straightedge and check how deep the depressions are. Then, you need to decide if it is in general level, or slopes from one side to the other. Then you can decide if you just want flat, or want to achieve both flat and level.
If you come back with some numbers, it's easier to suggest a fix.