You have a couple of options.  To make your own objects from scratch, you will need to use Gmax  which is included on disc 1 of FS9 or FSDS which is a payware program from Abacus. 

Either one is fine but Gmax is a better tool for scenery.  I know this now after having used FSDS to design scenery now for the last couple years.  FSDS has many limitations that Gmax does not.  FSDS is really better suited to making aircraft and AI aircraft models.  But either one will work fine.

FSDS is easier to use, but less capable than Gmax.  Whichever one you decide to try, just know that most likely, you will not "get it" at first.  It was a year or so before I got the hang of FSDS.  Some people, never "get it" and just give up.  So don't expect results right away.  Luckily there are help sites out there and especially for Gmax some nice tutorials on how to create stuff.

Gmax is more versitile, but FSDS has its points for scenery design. For large scenerys it is better. Example make a square polygon part in both programs. FSDS produces a single poly to do the job. Gmax uses 2 triangular polys to do the same job. It's the same story with every part not just the single polygon. Probably the reason why Gmax parts have a slicker look to them. Remember a 20,000 poly FSDS scenery if made in Gmax will be 40,000 polys. So each program has its strenghts and weaknesses, why not learn both. Gmax is basicly a free program, you already have FSDS. That way you can use either depending on the needs of the project you want to design.

Glitch

Hmmm... I stand corrected. Must be something I have screwed up in my Gmax settings. When I create a part in Gmax, what I want to show up as a single 4 vertice poly is drawn and counted as 2, 3 vertice polygons and that is the polys in all parts show up that way. I am just relearning what little I knew of working with Gmax. I just finished a 2 & 1/2 year FSDS project for my design group. Please pardon and discount my previous statements.

glitch