The best way to start playing Necrons in Warhammer 40,000 would be to start with a small, easy-to-build and easy-to-play army that will allow you to learn the basics of the army and the game itself. One option would be to purchase a Necron Starter Set, which typically includes a small collection of easy-to-build miniatures, a rulebook, and a set of dice. This is a great way to get started with the Necrons as it provides you with everything you need to start playing right away. Another option would be to buy a Necron Battalion box, which gives you a good number of models and a good variety of units that can help you learn the army and its playstyle. As you become more familiar with the army and the game, you can then expand your collection and start experimenting with different units and tactics. It's also important to familiarize yourself with the Necron codex and the rules for the units you have, as well as to read some battle reports and watch some videos of Necron armies in action, this will give you an idea of how the army is played and how to use the units effectively. Finally, you can also find a local gaming community or club and join them, this will give you an opportunity to play games and learn from more experienced players and also make new friends.