Replacing rear hub, beginners advice required
There's two general types of wheel bearings: cup and cone and sealed cartridge bearings. 

Cup and code bearings were very common years ago and are even still used on some high end Shimano hubs (dura-ace). http://sheldonbrown.com/cone-adjustment.html. These bearings are easily replaceable with minimal tools. If you have these bearings and have worn them down too much, they can damage the bearing race in the hub. In that case you'd need to replace your hub.

Cartridge bearings are more common today. (http://www.pinkbike.com/news/To-the-Point-Cartridge-Bearings-2013.html). The advantage of a cartridge bearing is that even if it wears badly, it's not going to damage the hub. The disadvantage is that they are pressed into the hub shell and require more (and expensive) tools to service.

I'd be surprised if your bike had cup and cone bearings, but the only way to know is to do a little investigation. If the races aren't worn, replacement is quite easy. If you have cartridge bearings, any bike shop can easily press out the old ones and press in the new ones.