you probably won't have any difficulty getting into the game and learning how to play. D2's user interface is, in my opinion, one of the most intuitive out there (you can play the whole thing with just your mouse, honestly).
We will be playing with the expansion pack.
Ladder is based on who levels the fastest. Whoever gets to the max xp amount first in his/her class is at the top. But no one really plays ladder for that. In Ladder on D2, there are extra special items and formulas that you can find/make that make for a much more fun game.
When the ladder resets, all ladder characters become non-ladder characters, which means everyone starts over from level 1 with new characters.