This is a tough one, because the Time Capsule has been designed to connect to a modem or another router using a wired Ethernet connection.
One not so desirable option is to set up the Time Capsule to create a separate wireless network and have your Mac connect to that network when it needs to backup.
This would mean that your Mac would have to manually "switch" from your normal wireless network to the Time Capsule network in order to back up. Once the back up is complete, then the Mac would need to manually "switch" back to your normal wireless again.
All things considered, you might do much better to simply buy a USB or FireWire hard drive and connect it directly to your Mac. That way, the Mac can stay connected to the WiFi network and the Internet, and the backups from your Mac will occur automatically if you are planning to use Time Machine or another backup application.
There is one other option that might possibly work for you.....but it is complicated. That option would involve buying an AirPort Express to "join" the WiFi network and then provide an Ethernet cable connection to the Time Capsule, which would be configured to provide your own private wireless network. In other words, the AirPort Express would act like a "modem" to provide an Ethernet connection and Internet connection to the Time Capsule.
That's the theory. "How to" do this is another matter that would involve having a clear understanding of exactly how you connect to the WiFi network now. This may or may not work, and the only way to really know is to try it out.
Not sure which way you might want to go at this point. Let us know.
