See how deep a shovel will go into the soil. If you have less than four inches of soil, you must add topsoil. You want to make things favorable for the grass and unfavorable for the weeds so the grass will choke out the weeds. If the grass doesn't shade the weed, the weed will shade the grass. Sun is food.
Food is strength and life.
Shade is weakness, disease and death. Grass will shade the weeds only if it is tall enough. When you whack the blades off, the grass has to RACE to make more blades to make sugar. It then grows amazingly fast. This fast growth uses up a lot of the grass's stored sugar, and weakens the plant.