So, nowadays, everything is pretty well developed, that we don't even call our fellow country-mates, "Civilizations," we are too developed to have worry to create another too.
So, what are the basics of starting one, if you lived a bit back in the day?
Well, it mainly depends when, but if we can just say anytime during the Middle Ages, which is most likely what you want to know, then I got your answer. (Middle ages *470 AD - 1500 AD)
Well first of all, you need to get yourself known that you are a civilization. Now, its a lot harder now to create your own nation, but its pretty possible. If they so felt like it, we can experience a second Roman Empire that ingulfs half the planet. But it just didn't happen recently. Back in the older times, you can probably start your Civilization easily without rules to live by, but you will have a lot of work on your back, where to start? Start with Agriculture.
1. Agriculture / Farming Edit
Depending on where you started your own Civilization, farming shouldn't be too tricky. Although if you are doing this alone with only yourself, then you probably wont succeed.
So grab a few workers or something from another Civilization nearby that you left from (Make sure your emperor allows this, or your in for a beating!) If they have their supplies, they can get started.