Newly-seeded grass needs about 3 to 4 weeks to get established after germination before you mow it for the first time. The blades of grass are tender and easily damaged. Also consider limiting foot traffic and mower traffic, as this could compact the soil, especially if the soil is moist. When the new grass is about 3 inches high, it is safe to mow.
Every grass has an ideal mowing height which varies during the season. Lawns are also cut according to specific heights for different varieties of grass. Typically, Lawn Care Companies will cut your grass how you want. Keep in mind if you do request your yard be cut at a shorter height, mowing your lawn too short can lead to problems such as weed invasion, drought and damage from the heat.