You can start by reading docs, reading the uber part of any guide, or my favourite:
SP: Get a character untwinked to the uber part (act 1 on hell/destruction) once you get there, just save that character file (so you can do it again once you have done them all) and then do them one by one trying to figure out whats the minimum gear required for them (this way you are forced to learn their mechanics instead of relying on your gear to carry you). Once you learn them, get better gear and thats it.
If you are on multiplayer, you can always just either use a fully geared char and just tanking/watching what they do, or store everything away so you can do them like you would from scratch.