I don't know what this is or what to do with it. It appeared after turning the soil to remove other weeds. It has purple underside to the starter leaves that first appear. I have additional photos if the attached is not sufficient.
It's best to know what weeds you have before tilling. Some plants can keep growing from little pieces of roots, so chopping them up can multiply them, not kill them. Like rhizomatous grasses. It can also bring buried seeds to the surface. I like to have mulch/compost ready to cover a newly tilled area, for the suppression of weed seeds, to prevent it from turning into dust, and to start rebuilding the natural layers of the soil as soon as possible.
Sorry, don't know what your plant is, somebody will.
This is a member of the nightshade family. Maybe Black Nightshade. A common toxic weed.