Grass seed needs moisture and warmth to germinate. Too little water will dry out the seed and too much will rot the seed. Soil temperatures should be at least 45 to 55 degrees Fahrenheit for cool season grass and 55 to 65 degrees Fahrenheit for warm season grasses.
Oxygen and light benefit grass seed and help it grow faster. Soil that is too wet usually doesn't contain much oxygen, so keep the soil loose and aerated. Plant the type of seed that correlates with the availability of light. If your yard is in the shade, plant a shade-loving grass, not one that needs sun.
Grass seeds need direct contact with soil to absorb moisture. Debris and weeds pose a problem. A thin layer of mulch over the seed can help retain moisture and keep pests like birds away. Keep the seed moist with light, frequent waterings until germination.