Hold windows key+r
Type cmd <enter>
Type ipconfig <enter>
Look for your default gateway (should be 192.168.0.1 or 192.168.1.1)
Type that into your internet browser bar.
It will prompt you for a password. You've likely never logged into it, so the default login id and password should be "admin" and "password", or "Admin" and nothing, or sometimes it's a bunch of gibberish printed on the sticker on the router.. Google your model to see what the defaults are.
From there you can go into some sort of parental controls and mess with timers, create a bedtime, restrict content, etc.
You may want to change the login id and password while you're at it, because if I were 18 I'd be Googling a way to log in and change it back.