Human enemies are only a problem if you have no pistol, just wait in cover and pop out when the try to shoot you, they are handy cos they always leave lots of ammo, if you dont have guns (? dunno why you wouldnt), use a molotov on them or a deo bomb.

Rifle ammo, you probably wont see until the jungle, but you should find shotguns in the supermarket, the rifle ammo recipe is also in the jungle in overpass (?) as a quest to find someone.

You will get the pistol and shotgun ammo recipes in the police station, one on each visit, which should then ease your ammo woes :)

Yes, pistols are very useful against humans, and always go for headshots. And also, if you have found a rifle and you are still in the city then chances are it isn't a very good quality one, but always useful to sell for cash when you do get better guns in the jungle.