Let me chime in here.
There are a few vital tools required for scenery design. ADE for airport design, SBuilderX for scenery design in general, as well as various 3d software if you want to build your own scenery objects. It is a good thing to go through a few tutorials and the SDK documentation that comes with FSX or P3D (I would recommend the latter). A good source of information and help, if required, is www.fsdeveloper.com or Sim Outhouse www.sim-outhouse.com , where I am the admin of the Prepar3D forum. A lot of good SIM people are on SOH and there is always help if something comes up.
Cheers,
Mark