well you better start now before it gets hot
If you want to do it on the cheap then buy a dethatcher attachment for your lawn mower run it a couple times if you have a tractor run it more then a couple times
then spread weed and feed this will kill your weeds
get a 5 gallon pump sprayer and spot spray big weeds
let the lawn sit about 10 days and dethatch again then
goto servicestar / agway and get a couple big bags of seed
cover the entire yard and dont walk on it much just keep it wet for about 2 weeks
then in about july you want to weed and feed again but very lightly then in the fall put down lime
every year you will have to spread Weed and feed fertalizer to kill the weeds and promote grass growth
also watering
if you water just a little that will make the weeds grow and the grass wont get enough water to grow
You wont believe how much you have to water to get the soil moist down to 2-3 inches It may take 4 hours overnight but it is a lot of water
so its best to buy grass that can grow in direct sunlight in hot climates and then dont cut it too often because that will require more water and the stalks of the grass as they grow high will shade the roots of the grass keeping them cool and in need of less water
other then that you can hire someone and expect to pay a couple thousand depending on what they want to do. but in the end next fall you wont have a better lawn even if you resod.