Q: How can I choose the items I want to delete from my storage in iCloud?

You'd have to go into the various apps on your device that store data on iCloud and begin trimming files.  For example, Contacts and emails that are stored on icloud, must be deleted using the Contacts and Mail apps.  Be aware that deleting data from icloud will also remove if from your device.

Backups:

Go to Settings>iCloud>Storage & Backups>Manage Storage; there, tap the device you need info on and the resulting screen lists Backup Options with which apps store data on iCloud.  Turn off camera roll and any other apps that might have a lot of data.  Then delete the backup.  That will free up space.  Photos should be regularly synced to a computer (like you store photos from a digital camera) using either USB via iTunes (on a mac use iPhoto or Aperture to move them to an album) or using photo stream. Go to Settings>iCloud and turn that feature on. 

Emails:

Regarding emails, on the computer, move emails from mailboxes in icloud to those on the device.  That can also save a lot of space.

Also see:  http://support.apple.com/kb/ht4847

Thank you -- appreciate!!