Before installing a new flooring on top of the sub floor, you have to be certain that it has been prepared correctly. Any improper surface will only ruin your new wooden, laminate or engineered flooring. Not only does the sub floor has to be in a perfect condition, it must also be properly level. Otherwise the unevenness will cause the floor to not last long and it will potentially damage your furniture too.
So the first thing you could do is of course to remove the old flooring, if there is one, from the existing area. This way it will be easier for you to identify all the imperfections on the ground. Always measure and level the sub floor even if there is no obvious problem when looking at it.
For small areas of unevenness such as minor cracks, holes, spots and chips, you can apply a layer of self leveling compound and use it with the primer. As for larger and deeper holes, put only a small amount of concrete mixture at a time and layer it out. Filling the big hole at once will result in cracking after it has been left dry. Also, do not skip on the primer because it is important to bond and protect the concrete, else the self leveling compound alone will not hold together properly.
After you're done restoring the surface, you still have to add another layer of underlay to protect your new wood flooring from moisture issue. Some of the high density foam underlays are able cover up very minor unevenness of your sub floor too as well as reducing noise transmission. Consider making a plan in advance, check the pricing around hardware stores. Choose a better flooring and underlay that your budget allows.