To delete all data you are syncing with your iCloud account from your iPad, just go to Settings>iCloud and tap Delete Accout. When prompted, elect to not keep the data. To delete iCloud from your phone but keep the data on it, first go to Settings>iCloud and turn all synced data (calendar, contacts, etc.) to Off. When prompted, choose to keep the data on your iPhone. When everything is off, tap Delete Account. Your data will still be in your iCloud account but your are the only one who can access it.
If you really want to delete everything there you'll have to go to icloud.com on your computer, sign into your account, then delete the data. For example, open Contacts on icloud.com, click on a single contact, press control-A to select all contacts, press the Delete key. Do the same for your calendar (select an event, control-A, Delete), your email, any iWork documents, notes and reminders. Finally, to remove all photos from your photo stream, click on your name on icloud.com, click Advanced, click Reset Photo Stream.