I dont know how to make aircrafts i just download them from here. I think it would be better to do that before starting to make your own.

But i may be wrong

There are several modeling programs out there designed specifically with FS in mind, although you can use any 3D modeling program that exports directly to a direct x file to model with.

Most common FS modeler programs:

Gmax is a fully-functioning, freeware tool that you can design with.  The learning curve is a little insane; I've spent the better part of 6 years using it, and am still learning stuff.  But it is much more versatile than any of the others, save 3DS Max, which is expensive payware.

FS Design Studio (FSDS) is a payware program specifically designed to create FS projects.  It's a little more user-friendly than Gmax, but it costs money and is a little more limited in what it can ultimately do.

3DS Max is the big brother of Gmax, or possibly the daddy.  It is extremely expensive, and has way too many features to just use it for FS.  If you were already proficient at design, and was making that a top priority in your everyday life, this would be what you would want to consider.  It would be like buying a Ferrari that you only drove to the grocery store.