If sparse, bare spots or lots of weeds describe your lawn, it is time for a little lawn revival.
Start by evaluating the current state of your lawn. If more than 60% is bare or filled with weeds, it is time to start over. Look at this as an opportunity to properly prepare the soil, select a more drought tolerant lawn or convert it into a no mow or low maintenance lawn or planting bed.
Overseed thin sparse lawns. Core aerate first or use a slit seeder to insure good seed-to-soil contact. For small bare spots use a lawn patch kit or make your own. Mix a handful of quality grass seed into a bucket of topsoil. Remove any dead grass and roughen the soil surface. Then sprinkle the commercial or homemade lawn patch over the prepared bare spot.
Keep the soil moist until the grass seed begins to grow.
A bit more information: Larger areas will benefit from the addition of organic matter into the top 6 inches of soil before sowing the grass seed or laying sod. Keep the soil moist until the sod has rooted into the soil below and the grass seed begins to grow. Consider overseeding the lawn to help create a more uniform appearance in the lawn.