On speaking about how to kill weeds in the lawn the best way is not to let the lawn weeds to grow. The best way to prevent lawn weeds is to not to let the sunlight reach the lawn soil. It means to select a place on the shade for the lawn. Also adequate fertilizer water, mowing and precautionary measures prevents the growth of lawn weeds. Also you should take care not to let anyone or any insect trample over the turf.
There are basically two types of weed-control. The simplest one is to take the weeds out by your hand or with a hoe. But it is not possible if the area is large. In that case you need to use herbicide. For applying herbicide you need to follow the prescribed quantity and time of application labeled in the product.
To kill the weeds of the lawn there are two types of herbicides pre-emergence and post-emergence. Pre-emergence herbicides prevent the seed of the lawn weeds to germinate so it should be applied while the weeds are about to germinate. Generally they are applied during late February or early March.
If the problem of lawn weeds still exists then you have to apply post-emergence herbicide. Post-emergence herbicides are of two types selective or non-selective. Selective herbicides look after some species of plants such as broadleaf plants (net-like veins and showy flower) versus grass plants (hollow, rounded stems and parallel veins) without harming anything. Non-selective herbicides look after all plants. Sometimes non-selective herbicides become the only option to prevent the growth of lawn weeds.