You can always find yourself fighting looters and bandits wandering the countryside in Mount and Blade II: Bannerlord, but that can become boring exceptionally quickly. A great way to disrupt that boredom is to become a mercenary for a particular faction in the game. If you see a faction you want to ally yourself with and have a good relationship with, becoming a mercenary is a great way to do it, and you earn some decent gold, along with renown, for doing so.
You will need to have become a clan of level 1, first. You can do this by progressing through the main story quest of the game. Once you have a level 1 clan, you’re ready to seek out any of the factions you want to work with. You can’t find a random caravan of the faction, though. What you need to do is find a notable figure of that faction. Most of the time, they’re out wandering the world with a large army by their side. You can easily pick them out by holding the Left-Alt button while you ride on your horse, and when you reach them, you can speak to their leader.
While speaking to your leader, you want to say, “Ther’es something I’d like to discuss,” to them. After that, you want to say, “I would like to enter the service of,” and the end of the sentence should have whatever ruler oversees that NPC or you have the option to offering your sword over to that person if they are the one controlling the faction. After you end the conversation, you can now see that all members of that faction, and their towns, now have a green lining around their names to show you are allied with them. Any enemy factions, and their cities, that they are at war with now have a red outline. You can freely attack the red territories and their villages to earn additional gold and renown. Every battle you take part in gives you 15 denar. You keep whatever loot and gold you obtain from that battle, though.
The mercenary contracts do not have an end to them. If you want to finish serving for that faction, you need to seek out another leader of that faction and declare that you wish to leave their service. You probably want to do this if you need a break to rebuild your army, or you want to focus on restoring your lands and focus on more social activities, especially if that faction is in a middle of a war.