If the soil is heavy clay, it's not worth your time trying to grow a lawn unless you undertake some major soil improvement steps. Perhaps the pH needs adjustment. All this can be learned by submitting a soil sample to your county extension office. Once you're confident the soil is suitable for growing grass, seed away. Don't apply too much seed.
Most people do.
Not enough seed is far preferable to overseeding. One to two seeds per square inch will result in a far better looking lawn by the end of the first season, and the next, than a more spirited application. Rake the seed in lightly after applying, but don't bury it down more than an eighth-inch or so. Some seed will remain on top, some will be just below. That's the look you want.