Looking for an inexpensive portable system that sounds good in a gymnasium?

[dontmovemeagain]

I'm a Zumba Fitness instructor and am in need of a sound system that works with an iPod, is portable, sounds good, and is loud enough for a community college gymnasium. I don't know much about stereo equipment, so do I need speakers, an amp, a system? Need to know the terminology in order to search for the right products.