You don't need a green thumb to build your own green house.  But the framing material is critical for your greenhouse to last, so consider your options carefully.  Wood framing retains the structure's heat, but will require refinishing, and may warp or rot.  Aluminum framing won't retain as much heat, but resists corrosion and keeps its shape.  A newer framing option is rigid steel-core PVC pipe, which can be used for shelving as well.