For example Tamiyo, the Moon Sage. Can I use the -8 ability even though she only has 4 loyalty? I understand it would kill her. Or would I have to get 8 loyalty first with the +1 ability?
Wizard's wording on the matter is thus:
It doesn't specify how those counters get there. It could be through +X abilities, proliferate, or even cool things like Doubling Season. But they must exist on the planeswalker in order to pay the cost.
First things first - planeswalkers don't have "life", they have "loyalty counters".
Unfortunately (or fortunately, depending on the side of the table you're sitting on), you can't play an ability with a negative loyalty cost unless the planeswalker has at least that many loyalty counters on it.
See here for this and other official rules for planeswalkers in Magic.