Assuming it's summer where you are, I'd wait till late summer, then
spray it with glyphosate (Roundup) to kill all the existing vegetation.
When it's dead, you can remove the large debris, then roto till it.
Get the soil tested and apply any lime as needed before tilling. Then
get it leveled out, and seed it with the appropriate grass in late
summer/early fall, which is the perfect time.