I have the following suggestions: Get this free Demo version which never expires and keep it as a tool for the future
available here http://www.abacuspub.com/fsd/catalog/s531.htm
This will give you an idea of some basics, but is only meant as a glimpse of what can be done.
RadarMan provided you with a link to the Posky Tutorials which were conceived with AdobePhotoshop in mind. Argueably still the industry standard in graphics editors but not cheap.
Here are two tutorials using PaintShop Pro http://www.calclassic.com/repaint.htm
You can get a fully working version of Paint Shop Pro X Trial here http://www.corel.com/corel/product/index.jsp?pid=prod4900067&cid=catalog20038&segid=8100002
Many are of the opinion that for the purpose of repainting aircraft, PSP will give you everything that Adobe Photoshop does. Of course it's much cheaper and requires less processing power.
An even cheaper alternative would be 'SERIF PhotoPlus 10'. Available here http://www.serif.com/photoplus/photoplus10/index.asp
Regardless of which graphics editor you are going to be using, you will need this little utility: DXTBmp
available here https://flyawaysimulation.com/downloads/files/2883/dxtbmp-dxt-extended-bitmap-editor/
All of the repaint tutorials that I'm aware of, were written some time ago - so, needless to say that all graphics editors have newer versions in the meantime. But that's not too big of a problem.
Good luck with your endeavour - drop us a line and let us know how you're getting on