If you already are, there are three ways out: ask to be released from your oath, be denied a fief you requested and dispute the judgment by force, or ask your husband to join you in a rebellion ( female characters only). In other words, you must not be part of a rebellion or a vassal of one of the current factions.
The first requirement of ascending to the throne is that you cannot be tied to another claimant. Also, it will give you a fair fight when you start a level 5 king is not going to be popular or effective, at least level 20 is recommended. This gives you a chance to make friends, who will become your vassals. When starting the game, it is best to first be a vassal of another kingdom, and when you are powerful, revolt and start your own kingdom.
Before you start your own kingdom, make sure you have a lot of renown, either honor or dishonor (depending on your liking and desired relations), and especially, a lot of right to rule.
