PoB has detailed guide with leveling trees, gear and skills for budget version to get started and this one.
https://pobb.in/jKPrrOYTVBD5
Video has very detailed explanations on how things work, you can also check the budget guide for upgrade paths.
https://youtu.be/JX4H1yCo2a8
I updated it quite a lot since the video, new endgame version is coming next week with some amazing upgrades on both defences and damage.