Prepping your personal supply storage is the first step towards being a prepper, however, if you ask me, one needs to be proficient in a trade of some kind, be it metal working, carpentry, masonry, gardening, or first aid. These are all skills that any good prepper should have at least a rudimentary understanding of, but you should also become proficient with at least one of these areas, especially if you are prepping in a group (family, friends, ect). That way you all have a general knowledge to aid one another with their chosen skill, but you still have a specialist in a variety of areas.
For example, my primary prepping group consists of my wife, a couple of my brothers, and my mother. We are spread between 3 houses, but we are still sharing ideas and plans of operations. My mother is probably the most skilled gardener I have ever known, and is able to grow a mass producing garden in a variety of environments and space restrictions. My wife is training on food preservation, gardening and water purification, my brother is a skilled blacksmith, and my other brother has been improving his vehicle mechanic skills. I have chosen to train myself in carpentry and distilling. By having multiple experts makes us more self sufficient. It also allows us to have expert sources of life saving information in situations where the internet or text books are not available.
I am also of the opinion that your education is never over, and that you will need to continue to refresh your knowledge on subjects you have already covered and become proficient. Think about it, you can never have too much survival information, and you never know when there may have been a new breakthrough in your chosen trade skill.
As important as it is to know a trade that you can use to contribute to you and your families survival, you do not want to neglect general training in other fields of survival. For example, my brother and I have both received medical and tactical training from our time in the military, and we are working with our family on these skills as well, making sure we are all well rounded.
So in closing, the basic skill areas that your trade should be preparing you for should fall into one of the following:
Medical
Food and Water
Shelter
Defense