Humans killing other humans can get high-level human players after you; some of the groups are dedicated to chasing down PKers and post bounties on them and stuff.
EmVee's advice is good. If you are having a hard time finding ammo maybe consider picking up melee skills. Axes pwn.
Def get Flak Jacket, at least 1 First Aid kit (other players often heal you, but you can't rely on that, as if you get infected you're ******), if you're going guns get a shotgun (can also be found in bars), if melee get an axe.
My scout is my best char:
Free Running, Melee Axe tree maxed, Construction, Bodybuilding, Firearms Shotgun tree maxed. He can run, gun, chop and build.
My cop got killed.
![Sad :(](images/smilies/frown.gif)
My zombie keeps getting headshot before he can level up. He wants braaains.