There's several tools out there you can use. The previously mentioned is one of them. A shareware tool called Gimp (
http://gimp.org) does a nice job and the price is right. I've done quite a few renditions using Photoshop Express, which can come bundled with hardware and other computer stuff. It's basically a slimmed-down version of the full Adobe Photoshop. Adobe Illustrator is a professional-level graphics tool and there's always Corel Draw. MS Paint, which is normally loaded on your computer when you get it can be used as well. Theres really a long list of tools you can use but here's the short list of my personal methodology:
1) Pull down Van's 3-view drawings and do a quick paper sketch of the concept with crayons, colored pencils, markers, sharpies, your call.
2) Use your favorite graphic tool to do a 2-D drawing. Here's what this looks like:
http://adap.com/rv7/RV7_paint_rcaf.jpg
3) Finally, do a re-skin on MS Flight Sim, with what-ever version you have. This step takes a little more effort and entails the use of some different tools, but when loaded into MSFS, allows you to get a really nice look at your scheme in any weather, lighting, or background. Most of my examples are here:
http://adap.com/rv7/paint.htm.
4) Finally, I'm using Microsoft Expression Design and trying my skills at profile drawings. This needs work, but it's coming along:
http://adap.com/rv7/RV303P.jpg
This is a little simplistic explaination, but the process works for me. I've been noodling on prospective schemes for the entire 6-year build process so this method allows me to triage bad designs early and allow focus on those that I really like.
Good luck!