Once you've recruited a soldier, you can use them in battle to gain experience, which can then enable them to upgrade to better troop types. Regular troop types seem to be more favoured as the game progresses than any of the bandit classes, so consider raising troops from their respective factions rather than using a bandit force. You'll need to either buy recruits from settlements for a small amount of gold, or convert prisoners into troops after capturing them.
Unlike managing your army, the initial recruitment of troops in Bannerlord is quite easy. Watch on YouTube How to recruit troops in Bannerlord
