Margins are simple, just set the margins where you want the image to stretch.. done.
If Tiling.. same thing.. set where you want the image to tile\double.
Best way to learn margins.. is trial and error.. every theme, every part of each theme is going to have different margins.
There is no hard and fast rule as margins are determined by the image you have used for that section. I typically start at 4,4,4,4 and adjust as needed.
As a newer skinner I wouldn't stress too much on the extras tabs, keep it simple until you have a better understanding.. If you need help with something specific, just ask.
Finally..
best tut in the world...open a skin that works the way you like and see how they did it.
Thats how most of us learn(ed) 
