Uh-oh! Help??

My laptop has a big red X on the battery image in the task bar. It says there's something wrong with my battery and I should change it; my laptop can shut down suddenly at any time. That's helllaaaa not good. o_O Does anyone know what I can do? Where does one buy a battery? How do I change it? Does it cost a lot?
I wouldn't even know where the battery IS in my laptop anyway!

(lol I keep saying 'my' laptop as if it's really mine!)