If you're wondering how to kill weeds in lawn, you have several options.  

Boiling water 

The most effective way to get rid of weeds in your lawn is to boil water and pour it directly onto the weeds. 

Weed-and-feed products are available in both liquid and granular form. 

Using weed-and-feed products 

Using acetic acid in vinegar 

Using pre-emergent herbicides 

Weed control in your lawn requires using a pre-emergent herbicide.  

Some of these include using boiling water, acetic acid in vinegar, pre-emergent herbicides...