Pick Roland, finish the Intro and do missions. Since the turret is weak first and depletes very quickly and takes 100 seconds for cooldown, you probably will get this in Skag Gully. If the enemies are a bit too strong (even Lv 7 Skag seems to be hard for your fresh Turret), just go back to main menu, load again and place the Turret in front of Fyrestone, the skags coming out of the cave is an easy boosting spot.
I had a few problems when i activated my turret in front of the higher-leveled skags, as it took me quite more uses of that.