Moss is the most common lawn problem of all. There's no single cause of moss. It could be any one, or a combination, of:
To get rid of moss, work out what's causing the problem, then do what you can to eliminate it. A proprietary moss killer can be helpful.
A good modern grass-seed mixture gives you a dense lawn with the ability to compete better against moss and weeds. Choosing a mixture for shady lawns, such as Turfline® Shadow, also helps limit moss growth. Or you could get your lawn off to a good start with a lime-coated seed mixture, such as Turfline® GrassFix, that helps prevent moss growth from the time you sow.