Whether you've neglected your lawn, you recently purchased the property and it's former owner neglected it, or maybe last year's drought has taken a toll on it, you've made up your mind that it needs help. The amount of revival required and the steps to accomplishing it depend on how much damage there is and how much time, effort and money you want to invest.
There are 3 levels of renovation (for more details click each of the links):
As you might expect, those 3 levels go from relative ease, to down right dirty. Which method do you need? Repair works for lawns that are in pretty good shape, with a few bare spots, that might need repair. Otherwise, a regular fertilization and weed control program should make a world of difference.
Renovation is for lawns that there's just no hope of reviving without years of care, and then it'll still be iffy. Or, you want to change grass types. Out with old, and in with the new. Renovation requires killing the existing lawn, but doesn't require tilling, grading or adding additional topsoil.
Reestablishment means replacing and fixing everything: all the old grass is completely removed (no need to kill it); the soil's surface is graded and tilled, new topsoil is added if needed (and it probably does) and amended with other good things like organic matter (compost), maybe lime, or other trace elements determined with a soil test. This last step is really best done by a professional, unless you're in to using some heavy equipment.