You don't absolutely have to take a town from your former faction--that's only one of the ways to start your own faction. You can renounce your oath and take a town from another faction. Or, if you're a vassal and have besieged and taken a castle or town and asked for it to be granted to you, if your king refuses, you can then revolt. That way, you keep all of your fiefs (except villages attached to castles/towns that you don't own).
A more complicated way to start your own faction is to take up a claimaint quest. Since you're the claimant's chief marshall, minister and adviser, they listen to what you say and you can basically assign all fiefs to yourself and then tell the claimant you don't think your rebellion will succeed. The claimant will go away forever and you've got your own faction with a lot of properties (and probably a few lords by then). Pretty devious, but it works!
Female characters can also revolt by convincing their husband that they should be queen; they'll start off with all their joint properties and their husband as lord.