Use a mosskiller - comes either in a liquid which you mix with water and water on, or wait till April and buy a Lawn Weed, Feed and Mosskiller product - granules which need sprinkling. With either, you'll need to rake out the moss once its blackened.

Also, lawn sand will do the same job.

Mosskiller would be the quickest result, especially if it is mainly moss but if it is not too mossy then aerating the soil with a garden fork/aerator, raking over it, then add lawn sand would be best and help longer term.

We were also advised by Greenthumb to raise the cut on the mower. Leaving the grass slightly longer certainly reduced our moss problem.