He was the apprentice. He learned first from Qui gon Jin (sp), then after he died, Obiwan taught him until he was seduced to the dark side and then the emperor trained him I would guess.
About how Palpatine becomes emperor, I am sure we will see it in the 3rd episode of Star Wars.
I think he became the emperor by intrigue, usual politics and starting the clone wars of course, slowly working his way to the top. He must have been pretty powerful if he could fool all those in the jedi high council. (nice job )
Nobody knows, nobody cares. The point is that somewhere along the line, the Sith master found his apprentice, Palpatine, who was so incredibly powerful that once he learned what he thought was sufficient, he killed his master to take over the title of Dark Lord.
Most is mystery between Palpatine and Darth Bandon. Bandon was the one who changed the ways of the Sith to only two- a master and an apprentice. You can see the messes that happen in KotOR when there are too many Dark Jedi in one place... they all kill each other instead of their enemy.
"No bastard ever won a war by dying for his country. He won it by making the other poor dumb bastard die for his country."
-General George S. Patton