HELP! I cannot figure out the privacy settings!!

We are brand new at this multi-site thing. We currently have over 300 users with individual sites on our other network. We are about to launch our new sites on the mutli-site network, but I cannot figure out how to set the privacy settings so that individuals only see their dashboard and not all 400 dashboards. I also want to set it so they can't mess with certain settings and break their site. I tried to set it up and ended up locking myself out of all of it. AMATEUR. I've watched videos, read tutorials, it's just not making sense. Please help.