The area the game is focused on is on the Med - The Roman republic, Carthage and the various Greek successor states. In EU 3 terms, everything else is the Cherokee and Creek nations from EU 3. Far away, there only to be conquered by the expanding European states, and by extension not very fun to play. Arguably the map should have been expanded further east and south at the expense of the cold, wet, damp and practically empty European north and west. Your gameplay experience will be vastly superior if you play one of the nations that are designed to be played. A tribal nation in England is not going to be fun to play at all as theres not much around you and not much you can do for a very long time.
Essentially though - reforming governments is done by decision. Meet the conditions and you can promote from a tribal government to a republic or monarchy. The conditions are specific, but in general terms your capital needs a civillisation score of 50 or more, Stability of 1 or more and your ruler needs a finesse score of 8 or more. Theyre stored in the "Conversions" file under Decisions folder.
The trick though is building up 50 civillisation when youre thousands of miles from civillisation. I'm sure its possible, but its uphill all the way.