Securing AirPort Access

I have iMac and just got a new MacBook Pro. Learning my way around it. My iMac is connected to internet using DSL and has AirPort base capability. When I turn on AirPort on the iMac, I can access the internet using my MacBook just fine. Question: How do I keep neighbors from using my iMac airport to access the internet through my DSL? I pick up several of their signals, all encrypted, so I can assume they get mine but unsecured. Do I have to set up my own network to secure access through my iMac airport for my MacBook and any guests I have in my house?

In AirPort Admin Utility, click Change Wireless Security, select WPA2 Personal and put a password into the two boxes showing.

Apply the changes to the base station and you should be set.

Regards