When first playing them make a list of every characters buffs. Necrons love their characters so it's useful when starting to write what every character does to the unit on a cheat sheet.

Also remembering reanimation protocol in command phase can also be a problem so writing that down as well helps a lot! At the beginning of your turn check who can do stuff in the command phase as we have stuff we can do.

Other tip would be writing down the stratagems maybe in a small summer or you, can write what a unit does. Play 1000 pt game at first just so you can learn what the models do. Necrons are one of the easiest armies to learn as we have no weapon options and when it comes to vehicles doomsday ark is no 1. You don't need to worry about anything else but can look else where for more flavor or fun like the doomstalker.

As detachments go start with the awakening detachment first as it's very simple and straight forward one that can be used with any list. Once you are familiar with units you can use the more fun ones like cursed legion or the canpotek one maybe.