Companions can be found at a tavern in town. They spawn randomly and each have their own unique skills and traits. If you are having trouble to find a companion or want one with specific skill-set you can visit a tavern and ask tavern keeper about the companion you want and for a mere price of 2 denars you'll get a specific town where to find that companion, if there is one in nearby towns.

To create a kingdom you need to have at least one fief under your control, clan tier 4 or higher and at least 100 troops under your command or split between your clan's parties/garrisons. If you have all three of the requirements fulfilled, head down to "Fiefs" tab in your clan's menu and assign one of your companions to be a governor of that fief.

So after the companion arrives to be a governor of the chosen fief if you decide to go to fiefs' keep you will find the companion there. Talk to him and you will be able to create your own kingdom.

Once kingdom is created talk to a companion you want to make into your vassal and choose the following dialogue option to give him a title of a lord. To do so you will have to pay 20 000 denars , 500 influence and give him a fief of your choice. Afterwards you will be able to name his clan and once that is done you will have a new clan under your rule with your former companion as it's leader. That clan will now operate autonomously, serving your kingdom.