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. WPA PSK TKIP is the most common security type in use. Make a note of the SSID or change it to one of your invention.
Go to wireless mode and ensure that it's set to a protocol matching your computer's wireless adapter and the printer's(mixed mode is a safe bet).
Disconnect ethernet cable.
Go to the computer's wireless networking setup screen and check that you can still connect to the internet. Select that network.
Apply the wireless security type and password to the printer's wireless networking setup screen.