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.