you have to go to a feast (usually held after a Tournament) or you have to befriend a lord who has a daughter, niece etc. and then ask them to "Create a lasting alliance with they're household" and then you have get the girl's relation up to about 14. Once you are there ask her if she thinks the two of you have a future together. Dependig on your persuasion, charisma (I think, and your relation level with her she will say yes or no. If she says yes then you have to ask her guardian and they will (if they like you) say yes ask you to pay a dowry. Then after ya pay it you are done

hope that helps!!!