First: Search for the cursed castle (in the surface, left or right from your start point), there's a man in front of the gate. Make plataforms , at least 3 sequences of them (you'll have to jump a lot to escape his attacks). Talk to him at night and select "curse". The Skeletron will appear. Destroy the hands first, and them the head, using arrows or magic itens. It's done. Be sure to have 10 or more hearts, it'll help you a lot!