Best place to start is Age of Empires I: Definitive Edition But that’s beyond the scope of this thread.
It doesn’t really matter and it’s hard to suggest without knowing whether that’s your first AoE game or RTS in general, but personally, I’d probably do all ‘Art of War’ challenges with at least a bronze medal. After that, you should be more than ready to enjoy campaign content.
When it comes to civs- Franks seem to beast strong and easy to learn, before release most people would say English are the most user-friendly for newcomers. Both should be good choices imho.
Hello! Welcome to the community! England and France are considered very good factions to start with. If you need a quick guide on how to get started in the first age and making a seamless transition to age two, here is a handy video: https://youtu.be/OnKeQ4PGsxA