|First, I would reset the router to it's factory defaults. You do this by plugging it in and then using a pen or paperclip, hold the reset button on the back in for around 10 seconds. Release the button, let the unit reboot and then you have to plug your internet connection into the WAN port of the router.|
Once you've done that, plug a computer or laptop into a LAN port and open a web browser. Inisde the URL box of the web browser, type in the default IP address of the router (you can find this info in the setup guide/manual). In the case of Linksys, they tend to be 192.168.0.1.
Once you're connected to the management interface you can then make whatever setup changes you need to.
With regard to wifi, you may have to enable it. Once enabled, you will likely have to setup a SSID and encryption. You don't have to, but I strongly recommend using WPA encryption on your SSID with a strong password.
Once you have that done, your wireless client needs to scan for available wireless networks and once it finds yours, you have to enter the password for the SSID.