I know it,
I read an article about CAD software a while ago.
But I can only suggest NOT to use sketch up!
Why?
- You can't export to bgl (fs scenery format) as far as I know.
  You'd have to export it to gmax (if possible?!) and from there to bgl.
- You don't have 2D views, only 3D, which doesn't allow accurate designing!
- You also don't have enough tools/modifiers or other possibilties to alter your objects.
- Sketch up is not able to make basic objects with a high poly number,
  and you don't want to have a pentagon as helicopter rotor, or a square as aircraft wheel, do you?

I myself can suggest GMAX and FSDS (v.3). They are both very good,
I already worked with both, but at last decided to FSDS.
Gmax is free-ware (look at your FSX DVD or your FS2004 CDs for it.).
FSDS is payware, but for Gmax you'll need more time to get familiar with it.

I suggest you try out Gmax first (and read at least two good tutorials ), and if you don't like it, take a look at FSDS.
I can't say what's better, both have their good and their bad sides, but FSDS is definitly more beginner-friendly.

This should help!
If not, ..... 

-The Revelator

thanks for the advice i have allready tried both but can't get them to work and can't use them dispite tutorials.

What scenery? What for? 
I can make animated hangars, towers (look at my sign ) and other buidlings like that,
but texturing is not my best....
I don't design things for a boring place like house in sudan... 
So when answering me, make it sound great, stunning, unbelievable, just as if you were an advertising manager! 

hmm i can't answer you to this now, not much time.
but what problems do you have?
if it's parking, taxiway etc. you can edit the airp. with afcad (@ simv) or afx (payware)
if they are buildings the problems I donÄ't know...