It's best if you have something like PhotoShop, but
Paint dot net can get you quite far in the graphics end of things. The "easiest" way to get started is to open an existing "skin", and see how that one is layed out. Since a goodly portion of SH3 skins are embedded in the dat files, you'll need to download
Silent 3Ditor to open those, and then find the "Embedded image" that you want, and export that to be able to open it in a graphics program. You might also find skins in the ship, plane or U-Boat's folder, or in the Textures sub-folders. In SH3, most of those files use tga extensions. There are other people on the site here that know much more about the subject, and there are a few "gotchas" that you can stumble into...