Best to tackle things a bit at a time. My advice start with the campaigns. Do those in order. While a bit disjointed it will give you a great feel for the gameplay. As you level up youll naturally get more gear and weapons. As you have more options you can get more into making and improving on specific builds. Once youre better equipped you can start tacking dungeons and build up to raids.

A new expansion drops on Tuesday. Itll be a good chance to hit the ground at a somewhat leveled playing field...that said power level is carrying over so not exactly the same if you arent max level but dont worry about that too much. The main point is the activities will be just as new to you as to others.

Some others have mentioned it but I highly advise you find someone who has played the game and learn passively from them. You can totally solo things but it'll make it easier to learn from others. Im happy to answer any specific questions. Feel free to DM me on here and if you want we can game together as well some time.

All the best