If you have no expereince in scenery design work you will need to do some study on the subject. For FS9 there are many tools out there for building scenery.
In order to make scenery in FSX you must have the Deluxe version installed with the FSX SDK and the SP1 SDK update from the FSinsider web site. In FSX they have a 'Mission Builder' section of the SDK which many are not aware allows a very simple way to develop and make your own custom scenery.
There is also a free software from Abacus that works with the Mission Builder in FSX and lets you place objects in the 3D world. You must register with real information to get the add-on but it is free.
http://www.abacuspub.com/catalog/s700.htm