Provided you are not a younger son who gained a title he did not inherit from his father, or a muslim ruler in a dynasty with very high decadence, the requirements for founding your own dynasty are:
* Your primary title does not have Tanistry/Elective/Elective Gavelkind OR you're independent OR your dynasty head is of a different realm
* There are at least 40 living members of your dynasty OR you are a legitimized bastard
* You are not the dynasty head, you are not married to any of his/her children, and none of your children are his/her close relatives
* Neither you nor your parents are close relatives of the dynasty head OR the dynasty head is not of your culture & not your parent
If all these things are true, you should be able to see the decision and thus the other requirements.