A healthy lawn can outcompete dandelions, so thicken the turf by overseeding and by proper fertilizing, watering, and mowing.
Pull dandelions while they're small, before they produce a taproot and set seed. Once the taproot has formed, you must remove all of it, since new plants can sprout from even a small piece. A dandelion weeder with a forked blade is helpful, or use a hand weeder with a bent shaft.
For chemical control, use a selective postemergence herbicide labeled for dandelions in turf.