There are 4 stages:
1. Encrypt your wireless settings
2. Change your router's password
3. Change and hide your wireless SSID (network name)
4. Restrict the MAC addresses that can log on
Only do one stage at a time and make sure that everything works before moving on to the next. It is also easier to make the changes from a wired connection so that you can still access the router if the wireless goes wrong. You may have to refer to your router's instructions to find the details specific to your router.
1. In your router's setup you will find a facility to encrypt the wireless network. If you have a choice of WEP and WPA, use WPA with the Private Key setting. Choose and input a phrase that you will remember. WRITE IT DOWN FIRST. Put the same phrase in the WPA setting for your pc's wireless adapter(s). Check that you can still use the wireless network.
2. Now change the router's setup password and, if possible, the log-on name (NOT your ISP's adsl log-on and password. WRITE IT DOWN FIRST. This will stop any intruder accessing your router's set-up. Test that you can log in after the change.
3. Go to the router's wireless setup pages and change the wireless network SSID. WRITE IT DOWN FIRST. There is normally also an option to hide (not transmit) the SSID. If there is, tick that option. Now log on with your wireless pc. You will find your network but there will be no network name. Tell your software to join that network and, when it asks, enter the SSID manually. You may also have to re-enter the encryption key. Make sure that you can join the network.
4. Every network device has a discrete MAC number. It is either written on it, or you can find it by opening a cmd window (Start - Run - cmd) and typing ipconfig /all The MAC address is listed as the Physical Address and is in the format xx-xx-xx-xx-xx-xx where x can be a number or the letters A - F. Your router may have a facility for restricting the wireless adapters that can get IP addresses to those you list (mine is under Firewall as MAC Address Filtering). Enter the MAC addresses of ALL the adapters in your system - outsiders will not then be able to join your network.
As long as you do this carefully and stage by stage you will have no problems and will keep intruders out.