there are various ways to secure it. if you have mucked up the configuration then I recommend factory resetting it as it will default to wpa2 encryption. you can access your router by typing in the gateway address in your web browser which may be 192.168.1.1 depending on your model and how you have it configured.
Note if you are NOT using wifi in your household, you should disable it altogether in the router. I encourage you to visit linksys' website and download the manual if you do not have it handy.