Exactly what @Thrag said. Find two kingdoms that are at war, and if you want an easier time at it pick the bigger kingdom (if there is one) and join up with them as a merc. If the kingdom is at war you won’t need to bribe your way into a castle to talk to a noble as all the nobles will be out and about looking for fights. Then you can figure out where their armies are by clicking on the Kingdom button then going over to the ‘Armies’ tab to join them.
You might need at least a couple of dozen troops to get much out of it. You can do that by fighting a few looters, winning some practice fights and tournaments, and some bandits or quests I guess. But the early game of hitting looters/bandits doesn’t need to take very long at all.