This page of the guide to Borderlands 3 has a walkthrough for Children of the Vault, one of the main missions. This is the first main quest. During this mission, you will learn the basics of gameplay. You will also participate in numerous battles - one of the enemies you encounter is a boss called Shiv.
You start the campaign by selecting your character. Each of them represents a different play style and has different abilities. Check the Characters levels and abilities chapter to learn more about them.
The entire adventure begins with a meeting with Claptrap. Your first task is simply to follow him. The robot will first show you to the character creation panel. Then, you will need to unlock the further passage. You do this with the lever located in the vehicle to the right of the barricade.
Follow Claptrap - you will learn how to crouch and jump, you will get to the chest with weapons. Take its contents. Then, your friend will go under the door of the crypt and begin to negotiate the entrance. You can search the neighborhood in the meantime, in order to get additional equipment.
At that time Claptrap will be captured by a large magnet. You will have to follow him, simultaneously eliminating enemies. Use the surrounding elements of environment as cover. Remember that you don't have a shield yet, so look around for loot, especially for syringes that restore lost health points.
Then, go towards the steel door. Shiv is waiting for you behind them. The fight against him should not be the most challenging. If you keep your distance, he will only attack with a shock wave. The direction of these is clrealy telegraphed by the cracks on the ground. The chest has several shields - you can take all (and then sell), but you can equip only one of them.
a More detailed description of the battle with the boss Shiv in Tougher Opponents chapter.
After defeating Shiv, you will reach level 2. You can choose one out of the three available Action Skills. More about them on the Character development page.
Finally, go back to your friend (you can safely jump down) and repair him. (the tutorial on how to resurrect allies). Lilith will appear. Talk to her. This completes the quest and starts a new main mission - From the Ground Up.