Answer: Your plan of action really depends on how much time, energy and money you want to spend. You could have the entire area tilled up, remove all the weeds and grass debris, and add lots of compost and manure and then reseed. Or, you could use a weed killer, then spread a layer of topsoil/compost over the entire lawn, and reseed. The existing grass will sprout up through it, and then you could reseed bare spots. Once you've got more lawn than weeds, you can help promote a healthy, thick lawn, which will crowd out most weeds.
A healthy lawn requires regular maintenance; watering, feeding and mowing. Deep watering promotes deep roots, which makes the grass less dependent upon frequent watering. Depending upon weather, you'll want to water once or twice a week, leaving the sprinklers on for about 15 minutes per area. Feeding is important; plan on using a 3-1-2 ratio of NPK in April, June, September and late November or early December. Home Depot generally has the right type of fertilizer on sale at just the right time to apply it in your area. As for mowing, the general rule of thumb is to mow as often as necessary to keep the grass blades about 2 inches high. And, to cut off no more than one-third of the blade. So, when the grass is 3 inches high, it's time to mow. If you allow it to grow longer and then you scalp it, the grass takes longer to recover and you're likely to develop bare spots in the lawn.
Hope this guides you toward having a lush, thick lawn.
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