Look guys, this isn't all that difficult. Na2S will, of course, dissociate in water. Now, what has every chemistry teacher ever said about the anion of a weak acid?! that's right, it acts as a base. This is just a very simple Bronstead-Lowry acid-base equation (remember, a bronstead lowry base is something that accepts a proton, such as ammonia).
S-2 + H20 ====> HS- + OH-
Here, S-2 is a base (accepts a proton), with it's conjugate acid being HS-; H20 acts as an acid (yes, it can do that, since it donates a proton), with it's conjugate base being OH-.