Go easy on him Requiem, he's just a young'un :laugh:
First off, use the search function like RequiemDeath said. Tons of these threads are made and each one contains pretty much the same stuff.
And second, here are the answers to your questions:
How: Find Keys (Terror, Destruction, Hate) from the following bosses in hell difficulty: Countess, Summoner and Nihlathak. Each one drops a specific key and you need to cube 1 of each in Act 5 to open a random portal to fight a souped up version of either Duriel, Andariel or Izual. When you kill them, they will drop an organ (Brain, Eye, Horn). Get all 3 of those and cube them together (still in act 5) to open the Uber Tristram Portal. Inside are the 3 Uber brothers. After killing them, your reward is a Hellfire Torch.
As for builds to do the Uber runs, smiters are by far the most popular. Although you can do it with other classes too.