As the GM, I will handle all attack, damage, skill, saving throw, etc. rolls. You post your character's intentions, and I'll handle the mechanics of it.
I don't mind the hammer and nails, though it seems odd to have worn a tool belt to a bar. Perhaps your guy was there to fix something? That would work. As far it goes as a weapon, I don't see a hammer being any better than an improvised club (two by four, etc), so I have no issue. Had you pulled out an SMG, we'd have had a problem.
I'm not going to give away much yet - you'll have to learn how things work as we play - but I will state that, at any time, you may attempt a headshot in combat. A successful headshot (at least, one with a firearm) will result in an insta-kill of any zombie. A normal hit, on the other hand, will have to result in enough overall damage to take them down. However, do note that headshots are significantly harder to achieve than are regular hits in combat... something to keep in mind.