For the starter location, other people will be better than me. It may be hard to begin as tribal... I dunno
One thing you can do when beginning with Gavelkind succession law is conquer the majority of the counties of a de jure duchy and create the duchy title. This way, you will have to give some counties to your sons upon succession but they will remain vassals (and can be called to war). Care if you got 2 duchy titles! upon succession, you will have to give one of them to a son and this son will become independent (he has an equivalent title as you heir). Better not create the second duchy title if you are close to die.
Second, as Ireland in Charlemagne bookmark, your government type is tribal. You'll have to focus toward reforming as feudal (increasing tribal organization law to the max, improving your tribal holding, I don't recall which building... I don't play feudal ATM). When this mill be done, you'll be able to change your succession law.
edit: for the Tanistry succession law (only open for Ireland and Scotland I believe... maybe Wales too...), I often heard that it was a very good succession law but I don't know how it works and how to get it.