airport extreme bridge?

[don't do it]

ok i am trying to get my airport extreme to receive a wireless signal and send it through a ethernet cable to my computer. how would i go about doing this?

 

[don't do it]

or would it be easier to set it up as a modem itself?

 

[spaceballl]

Is your main router an Airport device? Or some other device, if it's an airport device then you just need to set the checkbox to allow the network to be extended. Otherwise, you can use WDS mode. I've linked to some relevant threads with info.
Here's one.
And another