What are the best classes in PoE?

There is no good Answer to that Question. In Path of Exile the Build is way more important than the Class, because each one can do anything. You can have Witches with Axes and Marauders casting Spells.

How do you get good at PoE?

Path of Exile is a very complex Game, the best way to get good is to follow Guides to learn the detailed Mechanics. After a few Hundred Hours you’ll be ready to make your own Builds.