Aha, did you see what I said in one of my last screenshot postings?
There are 2 main scenery design programs - Airport for Windows by Tom Hiscox and Flight Simulator Scenery Creator (FSSC) by Derek Leung. Most people including me on the forum prefer FSSC, but download both and play to see which fits you the most snugly.
You'll also need a program to create your own objects (buildings and stuff). For that you'll need Easy Object designer (EOD). You can use loads of 'other people's' textures that are already in your FS2002\TEXTURE folder - you'll be amazed. But when you get a bit more advanced you'll need a graphics program to begin making your own. I use CorelDraw but you can do a lot even with MSPaint (that you've got free with Windows) and Pixia, another freeware prog which I understand is similar to PaintShop Pro (payware).
Let us know how you get and ... and good luck.
It's great fun and one of the most satisfying things about FS. I guarantee the hair will stand up on the back of your neck when you see your stuff in the sim for the first time!
BTW - there is a learning curve but there are loads of people only too ready to help around here. We all had to do it!

Roger