Most routers support MAC address filtering. You can enable this so that only those wireless adapters whose MAC address has been entered into the router's settings can access it.
To find the adapter's MAC address (Media Access Control address), open a Command Prompt window and, at the prompt, type:
and press Enter. In the details of the Wireless adapter, you will see the "Physical Address". This 12 character hexadecimal string in six groups of two is the MAC address.
In addition, there should be instructions in the router's manual to enable WEP or WPA encryption.