Hi feebee,
Read the thread entitled:
WPA Wi-Fi Encryption Cracked In Sixty Seconds.
Then if you want to have a little fun with your neighbor who is stealing your broadband:
See
Upside-Down-Ternet.
Since the Upside-Down-Ternet is for Unix/Linux and I surmise that you are a Windows user, your best bet is to pay attention to the first method which encrypt's your broadband and use either WPA2 or WPA with AES encryption in wireless routers (which has not been cracked to date). You need to find out how to do this for your specific broadband router, so contact your ISP on how to do this and get them to walk you through it and ask for the documentation or web page that has the information on doing it.
If and when you choose to switch to Unix/Linux then do the second approach - might as well have fun with your neighbors that steal from you, eh?
Update Edit: When you get up the courage to try Linux, see the thread entitled:
25 Linux tips for Windows switchers.
-- Tom