Navigate to /private/etc/apt/sources.list.d
Delete the file "cydia.list" (click EDIT on the top right, select the file, and click the waste basket).
Navigate to /var/lib/apt/lists
Delete all of the files in this folder except the one called "lock"
Navigate to /var/lib/apt/lists/partial
Delete all of the files in this folder
Navigate to /var/lib/cydia