you could use blender. it is a free 3d program. I started using it about 1 year ago and like it. I know a lot of people don't like its interface but I didn't have any problems. I'm using 3d max now and love it. I think its easier to use then blender but it costs a lot more.

You might want to have a look at google sketch up

http://sketchup.google.com/

Never used it myself but I have read it very easy to use.

I have been looking at it but from my use it is a little too simplified for what I want, it does some of the stuff automated which on occassion can be rather annoying and it often gets it wrong.

I was taking a crack at making a rounded ship and it just wouldn't fill the bits I wanted with render. I have decided to take another look at my autoCAD which is far more complex but also far better for what I want. I hope to be able to build and render a complete starship bridge in the next few weeks using it so fingures crossed.

I will post my results here first for comments and critisism.

I'd second the recommendation for Blender. It's free and does just about everything the major packages do.

You could also look into Milkshape3d http://www.milkshape3d.com/ which is a more community supported effort. It's build specifically for modding games.

There is also some offerings from Izware. http://www.izware.com/nendo/index.htm Nendo is a standalone modeler that has the simplest UI that I've ever seen. It's shareware, after 30 day's (I think) it's like $50.

AutoCad is not something that I'd recommend, for fun and exploration. Now, it's companion application 3dsMax is one of the standards, but will cost you a small fortune for a license.

Best of luck! Would love to see what you're coming up with.

Wings3D is a FREE modeller based on Nendo but it's nowhere near the caliber of 3DS Max or Blender.