First thing to do is to either go into a store or call Care and have a fraud investigation opened. If someone is changing things on your account without your knowledge then that is fraud. Change all of the security info on the account, PIN, Question and Answer, etc. and ensure that the email address on file, etc. is all correct to you. Do not use a simple or easy to guess PIN and security Q/A.
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.