How can I turn the Time Capsule into a wireless modem?

I have got my TC yesterday and was thinking to make it like a wireless modem. But after I have tried for a few time and it seems not to be acting like what I was thinking.
My problem is that I need the TC to connect to my existing wireless network so my devices can have internet access via connection of TC. Can anybody suggest me a way to do so?
Also I'm not so sure what is the "extend a network" mean?
Because I have done that, after restarting the TC, it wasn't even found from the Air Utility. And then I have to hard reset it in order to make it searchable as before.
Thanks.

If you want your TC to act as your wireless modem device, you should hardwire it to your existing wireless modem, then your TC should show up in Air Port Utility so you can configure it...

Your TC is a wireless router not a modem if you want to keep your existing wireless router you can set the TC up in Bridge Mode and auto DHCP .
You will get more help if you state your service provider along with modem brand and part number .