|username and password are request to access the modem/router configuration through your browser (by going to http://192.168.0.1 for exemple). The default username is admin and the default password is password.|
You should change these so that no one (already connected to your network) can change your settings.
This is however very different from securing your Wifi network to avoid people connecting or listening to it. This is done, through the configuration screen mentioned above, by selecting a Wireless Security Mode (I use WPA2/Personal with TKIP+AES). You will then select a passphrase which will be required to access your Wifi network.
Additionally you can also disable the SSID broadcast for your Wifi network not to appear when people scan for available networks. You'll then need to remember the SSID (its name) of the network when connecting a new device.
Finally, you can also add MAC address filtering where you specific which device can connect to your network.
Hope it helps