Hi.
Welcome to Dakka, and
40k.
You might want to get this post moved to the '
40k General' section, as it is more than an 'Introductions' post.
Yellow triangle in the corner.
You should be able get some Necrons quite cheaply, as they were in a starter box recently, and people will be selling off the models they don't want from that.
Longer-term though, which are you happiest painting? Necrons are considered a good army to begin painting with, due to their 'simple' painting schemes. Ad Mech can be more difficult to paint.
40k rules, unit cards and points are available now, for free. 10th edition is just released, and the basic information is on the Warhammer Community and
40k websites for download. Get the app if you can too. The rules, index cards and points tell you how to build an army. The app lets you same it, and should have the most up-to-date information.
https://www.warhammer-community.com/warhammer-40000-downloads/