(For specific necro questions you may want to visit the subforum for that class.)
First off you’ll want to decide what aspect of the game you want to focus on initially. From the sounds of it you’re wanting to do dungeons. Second thing to consider is whether you’re wanting to play optimally (as defined by the community), or do you prefer on doing things your own way. There are sites we can point you to if you’d rather work with what the community has developed otherwise there are plenty of theory crafters on the forums if you want help working out your own style. If you could provide a bit more information as to what you’re wanting to do in the game we can point you in a direction.