We'll assume that you will be using the iMac to set up the Time Capsule.
Move the Time Capsule close to your modem/router
Power up the Time Capsule and let it run a minute or two
Hold in the reset button on the back of the Time Capsule for 7-8 seconds and release
Allow a full minute for the Time Capsule to restart to a slow, blinking amber light status
Note.....this type of reset will not affect any of the data that might already be stored on the Time Capsule
Next, connect an Ethernet cable from one of the four LAN <--> Ethernet ports on the back of the modem/router to the WAN "O" port on the Time Capsule.
On your Mac.......
Click the Wi-Fi menu at the top of the Mac's screen
Look for a listing of New AirPort Base Station
Click directly on Time Capsule......(or it might say AirPort Time Capsule)
When you click on Time Capsule, that will start up the AirPort Utility setup "helper" or "wizard". The wizard will take a few seconds to analyze things and then display a screen that looks similar to the example below, except that you will see your Time Capsule pictured.
Network Name.......Type in a short name for a Wi-Fi network. Later, the network will be turned off.
Base Station Name.......Type in a short device name to call the Time Capsule
Password.......Type in a password for the Time Capsule and write it down
Verify......Type in the same password again to confirm
Click Next
The setup wizard will set things up for you automatically. When you see the message of Setup Complete, click Done. Close the windows.
Click the WiFi menu again at the top of the Mac's screen and make sure that you are connected to the modem/router WiFi network......(.not the Time Capsule network.)
Post back on your progress and we will continue