Your router needs to support verbose logging. It's a mode where you can see all the technical details regarding your router (maintenance, errors, connection attempts, PPPoE connection, firewall, etc).
You won't have the possibility to see what other users did while they were connected to your WiFi, for that other applications have to be installed.
Log in to your router, then access the Administration area and enable details logging. If there is no option for this, then the router doesn't support detailed logging.