There is no campaign, just a big sandbox that you can tune to your liking

To start off, I would highly recommend sticking to a small map, and just playing against one easy AI until you get the hang of the game. Multiple AIs intriduce quite a bit of extra complexity and can lead to lots of frustration for newer players

If you haven't played the tutorials yet, you should, they'll give you the basics at least

Any questions, feel free to ask! Hope this helps