Maps and skins are hard to compare as they are completely different things. Is there something you would want to focus on first?
As for mapping I can only say: Just start. Drop your fav song into the editor and within the time you'll improve. Look at maps you like, ask others to take a look over yours etc.
Skinning: If you go back to the skinning section
here, at the top you may find a few useful posts like tutorials and FAQ.
Also
osu!wiki