Moss grows in the lawn because conditions are suitable for it - these are most often a result of poor drainage, compacted soil or heavy shade so the grass doesn't grow as strongly as it should, letting the moss get a foothold. If you don't treat the cause of the moss growing, it will keep coming back however much you treat it.
Once you've established the cause (often compaction/poor drainage which can be alleviated by aerating the lawn - stick a garden fork into it about 4-6" deep every 8-12") treat it with lawn sand or a proprietary weed & feed lawn care product.