I have a AT&T DSL modem router combo. How do I disable the router part so it will not interfere with Airport?

You won't be able to disable the "router part" of the modem/router that ATT has provided, but you can usually turn off the WiFi function on the device if you don't need or want to have two devices providing a WiFi signal.

Without knowing anything about the ATT product, your best bet is to check with ATT support if you want to turn off the WiFi.

Typically, you will enter the default IP address of the product in your Internet browser to get at the settings for the device.......something like 192.168.1.254......for example. Then, you will be asked for the user name and password, which is usually on a label on the bottom or back of the modem/router.

What are your networking goals with both the AT&T-provided modem router combo (or gateway device) and your AirPort base station? For example, do you want the base station to extend the wireless range of the gateway or to use it as you main wireless router?

Which exact make & model is the gateway and which exact model is your base station?