It depends on which spell type it is. To add normal spells, usually you just need to copy the abiltiy. Trigger enchanted spells; Copy the abitliy(s), custom units (and custom doodads and stuff too), and of course the Triggers. Jass spells; Do as with trigger-enchanted, i know nothin about jass, so i can't help you with that... This spell would probably be Trigger-enchanted.
EDIT: Credit is noticing the a name in the map (often in quests or loading screen). People who has helped you, authors of models, skins, spells etc. Credits are highly appreciated, because of the time the spell takes to make.