Sounds like your soil needs some serious help so the best solution is to add 4 to 6 inches of topsoil and till this into your old lawn and start fresh. Okay, a lot of work and expense so a Band aid solution is to aerate the entire lawn first, then add lime, then rake a few inches of topsoil over the entire lawn along with starter fertilizer, covering both old lawn and bare spots. The best time to add new seed is September or April in your area. Plugging the lawn with a core aerator is the best so hire this done by a professional to save your back and leave the plugs on the soil to break down. Buy the best topsoil you can and tell them it is for a new lawn not a vegetable garden. You want sandy loam. It helps the seed if you cover it with peat moss but some new grass seeds are now sold with a bit of mulch included and these are water saving grass seeds worth the extra money you might pay. Don't worry, you will have a fresh lawn this coming summer, and by the following summer your new lawn will be thick and able to crowd out most weeds. Keep growing, Marianne Binetti