Three things immediately (aside from investigations, which will take decades):
1. Start the process to expand the supreme court. Undo citizens united and the Dobbs decision.
2. Pass voting rights legislation and laws to restrengthen bribery/corruption accountability by public officials. (There are probably recent supreme court decisions to undo here too.)
3. Defund ICE/dept of homeland security. Release everyone from the concentration camps. Shut all of the camps down.