It is not difficult to set up a router, so you can easily reset the router to factory default settings and set it up yourself. Look at the back of the router and you'll probably see a small hole labeled "Reset". Stick a sharp pencil or straightened paper clip into the hole and press and hold for about 10 seconds. The router should reboot, and once it comes up it should be back to factory defaults.
Then, connect a computer to the router, either via ethernet, by using one of the ports labeled LAN or if you are unable to do this, wirelessly (the name of the network should be linksys). It would be preferable to connect via ethernet, so you do not need to keep changing your wireless settings on your devices. Then, open a web browser and try these three addresses:
http://192.168.0.1
http://192.168.1.1
http://192.168.1.254
One of those should get you to a login screen. Usually the default username and password will be either admin/admin or admin/password, but if those don't work just look for the model number of the router (probably on a sticker on the bottom), and search Google for "linksys [model number] default password" and you should be able to find it easily.
Once you log in you will get a control interface that lets you configure the router. Look around for Wireless Security and you should find a screen that gives you the option to protect your network with a passphrase. Set the security type to "WPA2-PSK" and come up with a strong password. Do not use the default password, as he may have noted this. Apply the settings and the router should reboot. This should be sufficient to prevent your neighbor from connecting and seeing your network traffic. You can now connect wirelessly.