Connect the pc to the router via USB or LAN.
In a browser type 192.168.0.1
It will ask for a password and user name.
The user name is generally admin but the password can be password or anything you set it to.
If you lost the acess password to the router setup(not the wi-fi log in) you can reset the router to default setting by turning on the router and pressing the reset button(its very small,you'll need a needle or something very small to press it) until the lights of the router turn off.
After the reset the user name will be admin and the password will be password.
Type 192.168.0.1 on the browser and input them.
Set the wi-fi on and set a password for it and you are done.
P.S.: Some router can be acessed throught 192.168.1.0 or something similar.Refer to the user manual to see which adress it is or tell us what router brand it is so we can look it up.If it is a modem-router from your ISP look at the back of the modem-router and it will say the Default adress(192.168.x.x),user name and password.
If it doesnt contact your ISP for this information.