Early on, you'll want to target castles as even with smaller defending armies capturing one does require quite a bit of effort.
Once you've done that, you're free to select a castle or town to siege. Just raid one of their villages which, aside from pissing off the owner, also yields loot for your army. Getting someone to hate you enough is terribly simple, though.
In order to do so, you'll first need to be at war with another faction. The direct method of getting a settlement in Mount & Blade II: Bannerlord involves, as you can probably tell, using brute force to siege it and defeat its defenders.
