I recently graduated, and accepted a job offer as an Executive Assistant to a Senior Manager, well the problem started when other staff in the office started asking me to help them out, and when they realized how I suck at saying NO, they now started going overboard with it, sending me emails to print, scan a document for them, get a file for them, etc

My direct manager does all these stuff on his own although he is more senior in position to all of them, and I've started to delay a lot of his work because of constant interuptions I've been geting.
What would be a professional way to tell them I'm busy and I don't want to be inturupted without beign rude?