When we bought our house I had the same issue. I tilled the yard, raked the yard level bagging as much of the weeds as possible then bought grass seeds(bulk) and a load of top soil. I spread 2" of top soil over the whole yard then raked in the grass seed. You will need to water like crazy for a while and keep the doggies off. Have your soil analyzed if you do not put new top soil it may be to acidic for grass(spreading lime will neutralize soil)