Connect the computer to the router with an ethernet cable.
See the literature which came with your router for how to access the user setup screens in the router. Once in go to wireless security and review or renew the wireless security type and password. Make a note of the SSID or change it to one of your invention.
Disconnect ethernet cable.
Go to the computer's wireless networking setup screen and see if you can detect the router's SSID (wireless call sign). Select that network.
Apply the wireless security type and password to the computer's wireless networking setup screen.