Bruce:
Here's a recent thread to get you started:
http://www.finescale.com/fsm/community/forum/topic.asp?page=-1&TOPIC_ID=27787&REPLY_ID=278683#278683
You'll need software of some sort to work with your images; decal paper made for whatever type of printer (inkjet, laser, etc.) you have; and some kind of clear coat to seal the artwork once it's printed. The decal paper manufacturer will usually recommend a suitable clear-coat.
You'll also need patience. I started with no particular aptitude either for art or computers, and it took time, a bit of wasted decal paper, and a lot of muttered invective to get the results I wanted---but when it worked, finally, it was incredibly worthwhile.
I've only been doing my own decals for about 2 yrs., and already I've done maybe a dozen "dream" projects that would have been impossible---or so difficult that I continually kept putting them off, due to the degree of difficulty of hand-painting or whatever---without making my own markings. The first time you finish a project that "no one else has," or for which no commercial markings are available, you'll be hooked. It's also a great way to "back up" existing decals, to be able to print replacements in case something goes wrong.
Hang in there. It's WAY worth it.
Greg