The easy answer is to use a broad-leaf herbicide for lawns. It kills plants with broader leaves but not grass, which has narrow blades. Follow package direction exactly.
But also keep in mind that the best offense with all lawn weeds is a good defense. Fertilize regularly. Apply a broad-leaf herbicide (which kills existing weeds) and a pre-emergent herbicide (which kills seeds that haven't sprouted) each year. (There are many lawn "programs" you can purchase at your local lawn and garden center. Ask a knowledgeable clerk about the right one for your lawn.)Keep it well watered, as much as is practical and environmentally responsible.And never let your grass get too long, that is, no more than 3 inches high. You probably should keep your grass between 2-3 inches tall, depending on your grass type. But don't cut it too short, or you can weaken it. "Scalping" a lawn encourages weed problems.