To get a character's Fury, you simply need to unlock the first ability on the left skill tree. Then, as you kill zombies a small rage meter at the top left of the screen (just left of your health bar) will fill up. Filling the meter can become easier through advancing the Fury skill tree, as each character has passive bonuses that can be unlocked that make it fill faster or just take less rage to fill the meter. Each character's Fury also lasts longer and becomes more powerful as you progress through their skill tree.
To use your Fury, hold B for a couple seconds, until the screen changes and you pull out a character-specific weapon. Hopefully you can figure out what to do at this point. It'll take a few tries, as Fury doesn't last very long, and there are no locations where you'll find 100+ zombies at once.
As for actually getting kills, running around the the City of Moresby can easily round up 6+ zombies, and if it lasts long enough you can sometimes get to another group of 3 or 4 zombies before you run out. Either way, it's best done by accumulation throughout the game in 5 Cent's opinion.
Also note that all 100 kills do not need to be done with a single character.