Sound system stuff, like hooking up subwoofer...

So it started out with my friend…She had seen a backpack with speakers, and as the school “wiz kid” she asked if I could put speakers in the shoulder pads of her new backpack, so she could plug in her iPod. Easy, just simple wiring. Then I got thinking, and that’s when things get complicated. I thought “Why not put a subwoofer in your backpack too?”, except I have NO CLUE how to hook up a subwoofer with speakers. I need info on small, non power hungry subwoofers, how to hook it up with a 3.5mm audio jack, possible a cheap small amp, and any other info you guys think I could use.