Open some skins in SkinStudio and study the settings. Learn what they do by playing around with them.
The way I learned SkinStudio was by copying a skin, changing the graphics around and messing with the settings. Of course, going about it this way means that you won't want to upload whatever you come up with since it will be based on someone elses' work.
My first 'skin' was done this way. I never uploaded that one. Never really finished it, either; but it got me going in the right direction.
Good luck!
