Welcome to Tomshardware Forums . First of all, you need to enable your own password for logging into the router's page that nobody without your awareness can login to the router and can changes.
And as far as the matter is concerned about intruders or unauthorized people accessing your networking or using your bandwidth , you can encrypt your wireless network. For that you may choose the WPA or WPA2 encryption. If your router supports you can even use the 64 bit or high 128bit encryption keys which is in the format of AES and TKIP. This would be highly encrypted and its very hard to break this key. All the best!!!