You can tell if you need thatching by cutting a plug about 1-2" and looking to see if you have thatch between the soil and new grass line. 3/4' and thicker means you need to thatch. Most new grass seed does not promote as much thatch. Airate the lawn. Sometimes called "Plugging" That will help also. Then over seed with a blend of grass. Most grass blends are now Rye grass. Make sure the seed is for your area. Lastly fertilize.
Use a sharp blade on your mower so the grass blades do not cut ragged as that will make your lawn look white with the frayed edges of the blades.
Fetilize, Water, Mow...