Since you haven't told us what you did, it would be hard to tell you what you did wrong, if anything. You didn't even tell us what version of OS X you are using nor what model of computer.
I would start by opening Airport Utility. Select your Time Capsule and enter the configuration mode. Click on the Disks icon in the toolbar. Select Airport Utility Help from the Help menu. Search for "erase." Follow instructions on how to reformat the drive correctly. Also, see How to verify or erase a Time Capsule disk and Backing up with AirPort Time Capsule for the first time.