Find a way to dry the soil out a bit (reduce sprinkler run times, aerate the grass to improve drainage, reroute downspouts, etc), raise your mower's cutting height, and if applicable, prune up any low-hanging branches to increase light levels. Also check the pH. If it's below 6.5, apply lime. All of these things will make the site more hospitable to grass, and less so to moss. Tipping the balance in favor of the grass may do the trick.
in the short term, you can use moss killer. Most garden centers carry it. Use it, rake out the dead moss, then reseed if needed. Meanwhile, the steps above should help the lawn fill in over the long term. Otherwise, the moss will just return year after year if you don't solve the underlying problems.