Answer: You could kill all of the existing vegetation using a non selective herbicide containing glyphosate. Be sure to read and carefully follow all of the label directions exactly. However, the best time to start a new lawn is actually late summer to early fall -- not spring -- so I would strongly suggest you wait until then to do this.
In the meantime, you could work to improve the soil in preparation for replanting: run some basic soil tests to check fertility and pH, then fertilize and lime according to the soil tests. This will help your lawn get a better start next fall.
Your local county extension should be able to help you with the testing and interpreting the results and may have additional suggestions as to ways you can minimize the weeds this summer. Then when it is time to redo the lawn, they should be able to suggest the best ways to prepare the soil and the best grass seed varieties to plant in your local area.
In the meantime, you may find some helpful information at their web site:
Good luck with your lawn program!
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