Lets see a picture man, this is the best time to do work to a lawn
Heres what to do,
1. get a spreader and a bag of "scotts weed and feed" if you see a lot of dandelions. then buy the weed and feed that has dandelion control.
2. Any weeds that are left after the weed and feed application get some roundup and spray them.
3. Buy a 50lb bag of grass seed, typically what they sell in the stores around here will grow (lowes) or if you really want to get into it go to Sweeny Seed.
4. Take a hard rake and rake all the bad spots and remove all dead weeds, then throw down the grass seed Note; Put a shitload of seed down I mean cover the ground.
water an inch a week (at least), put a sprinkler out and a small cup in the sprinklers path, when it hits an inch your good
If you need further help call me, you gots mah number
P.S. I know some really good "Turf Geeks" as a matter of fact one of them is a member here, although I dont know her screen name.