Wait until fall or spring but in the meantime you can use a weedkiller to get most of the weeds out of the way before they go to seed. Store up compost orcow manure and when you till the area make sure they get mixed in and remember seed will germinate above 50 degrees so don't wait until it is too late or early. When you rake and spread the seed or sod put down some bone meal because they will help root growth and then fertilize the season after you seed or sod. Water but do not drown and the best time to water is after the sun goes down because the mositure will stay a lot longer. Cut the grass at the highest setting on your mower and patch when they show up or guranteed weed seeds will. After the first year fertilize in the spring and in the fall and don't not buy the cheapest fertilizer because it will be filled with sand which would be good for clay soils but much too expensive for fertilizer. If you put in a sprinkler systems water every third day for 20 minutes in the evening.