The earliest Era you can play is Colonial, Where you are a colony of the British Empire. Eventually as you play through the Era you’ll be given a task from Penultimo called Just Wondering…, this is will be after a series of other main tasks in a Mission, or randomly as you play through a Sandbox game. To complete this you’ll need a few different things. The First is have an Average revolutionary approval of 60, and then have a 60% of your population support the revolutionaries. This is pretty east to get since every time you complete a Task for the Revolutionaries you can choose to get Revolutionary immigrants. Choosing this option will boost both the above requirements.
After you complete this task you’ll be given the task Is It That Time Already? (this may come after another task during Mission games). Here you’ll have an option to buy your freedom, which will cost you $10,000 or fight for it. If you have the Fort or Guard Tower blue print unlocked then the war option will be much cheaper, if you don’t both options will be around the same cost. If you pay, then the game will immediately progress to the World Wars Era. If you fight you have 720 days to prepare. The fight is super easy. One Fort and a few Guard Towers will be enough to win the revolution and gain your freedom. Choose whichever option you want.