Kill 1 Smog.
After going through the Cemetary and finding Curtis in the junkyard, he will repair your pistol. On your way back to the road you will encounter your first Smog.
These guys are extremely slow and very easy to kill.
Simply take aim with your pistol, once he scoots close enough he will inhale and his lungs will expose (huge red/yellow things) simply shoot him in the exposed lungs, he will then cringe back, simply wait for him to scoot to you and expose his lungs again, one more shot and he will die.
If you are out of pistol ammo, don't waste your time melee fighting him as you will take a ton of damage, and don't waste your shotgun or rifle ammo either, its very easy to run from them as they are SUPER slow.