Answer: A vegetation killer such as Round-Up will kill all the growth in the area. It's a non-selective herbicide so be careful not to allow overspray to make contact with other plants and use in amount as directed on the label so excess will not leach through the soil and harm other plants or trees.
An alternative is to rototill the area and rake out the debris, then rototill again as new weeds emerge. When you're ready to plant your lawn, rototil a third time and rake out the weeds, then level and broadcast your grass seeds.
Best wishes with your new lawn.
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