On this page of the game guide to Borderlands 2 you will find the walkthrough description of one of the first main missions - Cleaning up the Berg. You will complete this mission while progressing through the Southern Shelf location.
Southern Shelf - Hammerlock camp
1) Starting point (teleport to Windshear Waste)
2) Hammerlock (they're beside each other)
After you reach the destination , follow the Claptrap towards the settlement. As you go lower, you will be attacked by Monglets. After you get rid of all the enemies, approach the main gate.
With the gate open, head inside and get rid of all the Bandits. At some point, a Brat Bullymong will join the fight. If you back out, the Bandits will start fighting with it. Once the area's clear, approach the gate in the south-west part of the camp and you will meet Sir Hammerlock. Give him the Claptrap Eye and wait for the robot to be repaired.
Now follow Hammerlock to the generator. Eventually, to the right of the generator, a Fast Travel will be activated. Speak to Hammerlock  to finish the mission (you will receive a Shield which will give you additional protection against damage) and be able to begin side missions.