For scenery design you use a program called Airport v2.XX, which can be downloaded here
Scenery design is pretty easy, you only need the static object files(called macros), some are provided with the program, others are available at flightsim.com. To place your airport at the corrects location you need the proper norh-west or east-west coordinats and elevation for your aiport, both of which can be found at http://worldaerodata.com
Heres what an design interface looks like, as you can see everything is pretty basic. Ohh yeah you will also need to exculde(cover up) the old airport when you put yours on top, which is acomplished with an exclude .bgl file which the program also makes. To learn more see the FAQ.