If you are using a wireless router or access point.
1 - Make sure your give it a unique SSID, DO NOT use default name
2 - Change the administrator password. Any good hacker already knows all the default passwords.
3 - Turn on WPA encryption. (If you don't have WPA, update your Firmware) In the very least enable WEP, it's not as secure as WPA but it's better than nothing.
This is, by far, not everything you can do to secure your wireless network, but it's a start.