It's pretty open ended how you accomplish this (banditry, political intrigue, monopolies on olive oil) but it's guaranteed to be a lot harder and less straightforward than it sounds.

You begin as a lone adventurer, gathering wealth, influence, an army, and maybe an empire. Go buy it, or at least try the free demo. Mount and Blade: Warband is a 2010 PC game and a redo of the original Mount and Blade.