it seems to me that this is the first time you design a roofsystem.
due to the braodness of your question, it is extremely dificult to correctly answer your question. there are many factors that need to be considered in choosing a structural system for your roof.
if it is going to be concrete then the best place to start with is aci code chapter 8. you could do it with a one way flat slab that spans 15 feet.
if it going to be steel deck on wide flange sections then a W10 might be adequate at say 5 to 6 feet o.c. depending on the deck type. the roof deck could be filled with a wire mesh reinforced concrete then you have the choise of designing a composite system or just simple beam with no composite action. the other choise is to fill the deck with light weight insulating concrete. you may want to check with you local provider of light weight concrete that could be mainly siplast, elastizel, and one other major producer I cant remember their name. if you chose to use the last option then you need to pay close attention to the deck boundary conditions and details required to transfer diaphram shear to the edge supports. also, you may want to check with your local building department for the Product Approval and a letter of acceptance of these products and their assembly technic.
