You have to give more information than that. We need to know how your going to play your army (I.E. gunline, vehicle spam) before we could give any direction. However, I can comment on the points part. 500 points is great for a beginner. It is a very limited number of units on the table. So you have time to focus on the the way the game is played instead of where a 2000 pt game you have so much to remember (I.E. unit abilities, which unit has which weapon, different movements, and many more things). So personally, I would say focus on a 500 pt list (and you can get list ideas in the Necron list section), so you can learn the game.
For Necrons I would start with the basics, first get the Codex and read it, then get a squad of Necron Warriors and one other thing that interests you (it may actually suck/perform under expectations) but as long as the "rule of cool" is in effect I would go with it.
Then play some games with what you have so that you can get things down, and chat with your local players as the meta may shift between places on what is appropriate for random games. Some might be Win at all costs, and some might be very laid back.
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