...that's again a bit tricky. I guess you are not familiar with all that stuff, right? That can take now a bit longer :/.
First: Hit the windows key + R, type in cmd, hit enter. A menu will come up. There, type in ipconfig, and hit enter. That will show you your IP adress. Is maybe something like 192.168.178.35 or similar (it is NOT the 255.255.255.255, it's the number above).
So, now, after you have your IP, then open your browser. Type in the first 3 triplets of your IP, so in the example http://192.168.178 and then .1.
(it might be the same like the standard gateway mentioned in the menu; can't check that atm, i have a rather unusual setup here; edit: As far as my network knowledge goes it should be the same)
This should, in most cases, bring you to your router, to the access menu. It might be secured with a password. If you haven't set your own, you might either a) ask the person who did that b) check the manual c) try without a passwort d) test some standard passwords like 0000, admin, administrator.
Then you have to look around yourself, unless you provide the infos about the router model (that might be worth as first step, so that we can look up if the whole stuff here even applies to the situation), so that i can check out the menus online.