You may find it hard to create textures using paint, most scenery designers require a slightly more sophisticated program such as paint shop pro or photoshop. There is a freeware program called the gimp -
http://www.gimp.org/ which is a free alternative. I'm downloading it now to see what its like.
Anyway thats off topic - if you can or have created textures in Paint then hats off to you! However you create them the final textures should be in 8bit colour format (256 colours) & have dimensions of either 128x128 pixels, 256x256 pixels or 512x512 pixels. EoD & FS will be able to handle anything along those lines. Shout if you need more help!
For FS9 the simplest way to create the ground is with AFCAD2 as wji says. This will allow you to modify & expand on the default easily & with great results.
Here are three examples of this & similar combinations.
The first is a quick little project i'm working on to see what the easiest combination of design is, - (Please bear in mind the buildings are my first attempts in GMAX so don't laugh..) The taxiways are created in AFCAD2 with buildings placed with ObplacerXML from Gmax.
This is a super scenery of RAF Lyneham with AFCAD2 base & objects made with EoD & placed with FSSC.
This last one is from Bristol which I made with FSSC - as you can see the taxiways are reasonably crude compared to the AFCAD2 style.
So take your pic!
Birdman.