How do i get down there?

See that opening near the bottom-right of your screenshot? Means you need to jump UP from below, which is actually part of the DARK AREA. But that's pretty far in the game and with only 4 hearts, you're not there yet. Try finding easier heart containers.

I now have seven hearts. How much health does the skeleton boss have or how many times do i have to hit him?

Giant Skeleton has 250 HP but I don't know how much damage each weapon does, probably 10~20 each strike or item? Try picking up shurikens somewhere in the Dark Area if you haven't already.