If someone asks you to make coffee, they’re assuming you can spare the time it takes to do so. Just let them know you can’t (and if you can, for god’s sake, don’t tell people! Find something useful to do, quick!). If it’s your direct supervisor, however, it’s probably best to just do it.
For more involved projects (“Could you make 50 copies of these handouts?”, “Could you type up these meeting notes really quick?”, “Could you put together this mass mailing for me?”), it depends who’s asking:
If it’s someone who’s not in your department and who’s not a superior :
“Sorry, I’m pretty busy right now.”
If it’s someone in your department or a superior who is not your immediate supervisor:
“Sorry, I’m pretty busy right now. You’d have to check with [supervisor] and see if he/she can spare my time.”
If it’s your direct supervisor, and such requests are seriously time-consuming and preventing you from doing your other work:
“I’m happy to, but as you know, I’m also working on Project X right now. I know you needed that completed by Y date - what’s your higher priorty?”
In short, always make your own work your primary concern. If you have time to help others, and it’s something that’s reasonably within your job description, great. But never put aside your own job to help someone else do theirs.