you first need to decide wether or not to use the network manager and its graphical interface.
assuming you choose to do so then
1. right click on its ICON
2. choose the "EDIT Connection"
3. GoTo Wireless Tab
4. choose the wireless network you wish to connect to , e.g. "BTHomeHub-1234"
5. click on the edit button for it
6. make sure the check box for connect automatically is checked.
7. remove all other un-wanted WiFi connections.
8. save and close
*** I'd advise to restart the service after completion to make sure changes take affect