Initially your tribe will have to match. Each kingdom will start out with around 2 tribes.
When you choose your domain and spawn into the world in expo you are taking over an NPC who lives in that town/county/duchy/kingdom. So you will have to choose someone who is actually living in that town. Most likely that will be a member of the tribe living in that area.
Once you perma die you can always start out as a different tribe and walk back to your homeland or you might get lucky and find a different tribe as a ward. However it should be said currently the idea is only the tribes found in that biome or the transition zones around that biome will be available as wards.
Title holders are limited in the tribal choice based on the geographic location of their domain.