You'll need two programs to start:
AFCAD2.1 by Lee Swordy (or if you want to expand and do both FS9 and FSX and get a terrific payware product "AFX").
Scenery Builder
AFCAD will allow you to place the rwy, taxiways, ramp areas, etc and scenery builder will allow you to produce a flatten poly to level the ground out so the rwy and everything else afcad related will sit properly on the ground.
Having said that, right after I post this, you'll get a long list of every other program that will do this and more from everyone else that posts here. Some will agree with my choices, others will have their own, some programs will do both. It all has to do with personal preference and what you learned on and feel comfortable with. There is no wrong answer. So you can take my advice or wait for the other suggestions and make your own choice