GMAX is one it came with your FS2002 or FS2004 disks
They are in the extras section of the disks.
But one word of caution GMAx is not what I would call a tool for a begineer. There are some good tutorials out there one comes with the GMAX install. But unless you have had some experience with 3d drawing before it is a little deep. About like the Grand Canyon or the Abyss.  But this is the tool that just about all of the designers are using for a lot of games. I made a awesome 3d house using this, But gave up on the airplane I just could not figure it out.
There is a good handbook that you can find at any descent book store that comes with a tutorial CD as well that I have heard is great.
Oh by the way I hope you have pleanty of hair on your head as you might loose a little using this program.

From what I've heard both GMax & FSDS have their share of pros & cons - having worked only with FSDS V2 I would definitely recommend it, although you do have to pay for the program.  It has allowed me to build a complete detailed aircraft which I am very happy with, although the project started way back in April - that's the same amount of time it takes to bring a human baby into the world!  Anyway, as noted above, be prepared for some hair-pulling, but I've found the people on this forum exceptionally helpful in getting around the "impossible" stuff.

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Which sim do you wish to create aircraft for? Until we know that there's no point in recommending anything.