what is the Besy way to start preparing to get into Office Admin?

After managing Office Admin for several years I would suggest you become  proficient in Microsoft Office Software, especial Word, Excel and Access.  These tools will make things easier in not only the work but the ability to be more innovative in possibly improving processes that are in place today in organizations. You need to be efficient and professional at all times because you are normally the first face or voice of the organization. 

Office Admins have good phone skills, computer skills, organizational skills, and customer service skills.  It would be good to start participating in your school's student body or help a teacher -- maybe sign up as a Teacher Assistant (Office Admins get ask to do a lot - they are the assistant-- would be good to get practice receiving directions and following through on directions)... other recommendations -- take classes to improve your computer skills... brush up on Microsoft Office - and learn how to create documents, use a calendar, create spreadsheets or create powerpoints. There are more- but this should help get you started.