first i suggest you learn the gui triggers and get used to them then gradually learn JASS. gui can be filled with leaks and not nearly as superior to JASS. if your able to understand JASS fairly simple than defiantly start with that.
JASS can make very complex skills, gui on the other hand is rather limited and harder to keep them from leaking depending on how complicated you want ur skill to be.
there are quite a bit of tutorials on them all, check around wc3campaigns.net and here. theres also a site called jassvault or something, there is a link somewhere in wc3camp or here.