Girdershade: The Nuka-Cola Challenge
Received from: Sierra Petrovita [Girdershade - Sierra Petrovita's house]
Solution: I would recommend that you start this objective soon after you've left Vault 101 and visited Megaton, because your job will be to collect certain items and you'll want to "unload" them from time to time. In order to activate this quest you must visit a very small village which goes by the name of Girdershade. The village is located to the south-west of Smith Casey's garage. Locate Sierra Petrovita's house once you've arrived at your destination. Talk to Sierra. You can agree to take a full Nuka-Cola tour (you would be rewarded with an ice cold Nuka-Cola) or if you're skilled enough you can convince Sierra to focus on explaining your assignment (screen). Sierra will ask you to deliver at least 30 bottles of Quantum Nuka-Cola, so she'll be able to fill her fridge. Ask her about the reward and if you're convincing enough you'll find out that in exchange she can reward with a schematic for a nuka-grenade.
Exit the building once you've agreed to help Sierra in finding the bottles. You are going to be automatically stopped by Ronald Laren, who will make you a counteroffer. Ronald will ask you to bring him all the bottles of Quantum Nuka-Cola you find, instead of giving them to Sierra. His initial offer is pathetic, because he offers the same amount of caps as Sierra, but if you have a high speech skill you can persuade him to double that. As for the bottles, they are scattered throughout the entire wasteland. Usually each primary locations holds at least 1-2 bottles and they can be found in strange places. Thankfully, they're easy to spot, because they're glowing (screen). You can speed up this process if you decide to take a short trip to Nuka-Cola facility which can be found in the southern section of the big map. You would have to deal with a lot of robots and mirelurks inside the facility. It's also a good idea to be on a look out for Nuka-Cola trucks, because they usually store more than one bottle. You shouldn't have any problems finding 30 bottles, because there are over 90 of them total, so you only have to find 1/3.
Once you've collected enough bottles go back to Girdershade. It's important that you decide at the beginning to whom you'd like to give the bottles. If you give enough bottles to Sierra you will not only receive bottle caps, but she'll also give you a schematic for a nuka-grenade. If you decided to give bottles to Laren he will go to Sierra and lie that he found them all. You should also know that you can continue this process even when this quest is over, but I'm sure that you'll find better ways to earn some serious money.