When starting a fresh lawn, you first need to kill off the old grass. Or, in some instances, you might have grass growing where you don't want it, such as landscaping or driveways. If you have pets, children or use the area frequently, you will need to kill the grass in a safe way, avoiding the use of harsh chemicals that are not safe when they come in contact with the skin.
Spray the grass with an organic herbicide that is stated as being safe for use around people or pets, or spray the area with vinegar. The vinegar's acidic makeup will typically kill grasses and weeds without adding any chemicals to the yard.
Cover the lawn with dark plastic such as trash bags that have been cut open to create a larger coverage area. The dark plastic will cut out all light. Without light, the grass cannot grow. Any grass that wasn't killed by the spray will die from lack of light, and the plastic will keep other grass from regrowing from the roots. This doesn't require any harsh chemicals.
Place rocks or concrete blocks around the plastic to hold it down securely. Leave the plastic in place for about a week to ensure that all grass is dead.
Remove the plastic and till the area to cut up dead roots and make sure that all of the grass is gone. Rake the dirt smooth to prepare it for whatever you are going to grow in its place.