Here's my two cents and I'm not an expert by any means. Just do a Google search for the diagram. I've never coached 8v8, but I think having three levels (backs, mids, forwards) is simpler to understand than four. Whether that's a 3-3-1 or 3-2-2, I don't think that's critical. Usually for U13 teams, you've moved on to more tactics and less skills to some degree. However for a rec team, I'd suggest doing just the opposite. I'd spend more time on skills (dribbling, passing, shielding, etc.) all under pressure. The chances that anyone scores off a corner/free kick is slim, so personally I think there is more benefit to working on skills that all players will use during the game than set plays that will be 50/50 most times.
Sounds like this is a team where most of the girls are playing for fun, so that needs to be the focus. Make the drills into mini-games that everyone is participating. Minimize the standing in lines. Keep them moving. Have fun yourself. Bravo for stepping up.