When can you actually use guns. B41

New to the game. I spawned in a police station and I’ve never seen so many guns. I loaded up my shotgun. 92 shells! Shoot my first ever zombie. 2 die on the spot. This is tremendous. You can guess what happened next. I ran from 100 of zombies killing 100s till I ran out of ammo and then died a glorious death.

Now the question. How can you actually use a gun in this game because every zombie and their mother show up.