If you want to specify starting locations and edit civs, you have to do it in the scenario editor. You just have to remember that there is a hardcoded limit of 31 civs + 1 barbarians, so if you want to create a brand new civ, you need to remove one that already exists. You would then need to create civilopedia entries and assign a picture to the civ you created. If you want more information on modding, you should post the same questions in the Creation and Customization forums. You could get a lot more detailed info there.