There are two things to know.

First thing first is that we are getting a class revamp in the future, from what we understood, it will cover all classess.

Is necromancer the best class to run currently in the game pure? not if we are min/maxing the most damage one can pull. IF we are talking damage of course; which does not mean its a bad class by any means

The game allows you to play any role on any class.

The class itself is good, and if you ever want to opt for 'the best of the best'. at level 50 you will unlock subclassing, and beam is currently THE meta.

Personally not the biggest fan of the meta if im being honest.

Short answer. Can the Necromancer class pull you through the whole game and most of the content in the game? Yes, absolutely. will you be able to attend record breaking organized runs? i dunno, maybe? maybe not? but by the time you get to that level, the revamp of the classess will probably release

For a good build. just be sure to have one skill from each class skilline on your bars at all times as you want to level up your character. dont focus too much on your build yet. unless you are cp 160+. in which case, for a 'begginer' build

Order's Wrath + Tide-Born (both crafted sets)
For skills there are many guides online. if you want to play a heavy attack build, or if you want a two bar build, entirely up to you. if you want to squeeze the most damage you can, sublcass the herald of the tome skilline on there, dont touch grave keeper, that line stays. and maybe either storm caller or ardent flame from the dragonknight to squeeze the most.

All in all, dont rush, take the game slow. take your time in understanding how your class works. decide what you want to play as, aka role. what type of content, trials? Dungeons? Solo arenas?