Spring is a great time to plant grass seed. So, take advantage of the mild weather and abundant spring rains to reseed the bare spots in your lawn. Grab a garden trowel or fork to loosen up the soil of each spot, then gently fork in an even layer of seed. Don't bury the seed. Just gently work it in so it is barely covered by soil.
Some companies make lawn patch products that combine seed, mulch and fertilizer.
Those work too. But I find this simple method to be just as effective this time of year. Remember, your newly-planted grass seeds won't sprout if you have applied a crabgrass preventer. Make sure you choose the grass seed that works best in your climate and yard. Shade mixes are available as are mixes for full sun and high traffic areas.