Mini-DVI to VGA or Mini displayport to VGA. It's displayport if it's a newer Macbook, but you should google these two terms to determine which you have. You're unlikely never to need to adapt to HDMI, DVI, RGB, RS232, so invest in whatever you have to VGA.
edit: unless you plug into your home TV or something.
edit2: you can buy these things at Titles, helps to have your computer with you if you're a computer tard
edit3: you actually know it needs HDMI? Are you 100% sure of this (sorry, I'm tired)