Really all you need to know is.
Just mess around. Learn your photo editing program. I have self taught my self over the two years i've been repainting. alot of the stuff i have learned has been from youtube tutorials for photoshop. but once you learn how to use your program, you'll be learning more stuff left and right. 
But all you need to do is just use it. and just gain experience. no, your first repaint wont be the best. But. It's a start. but, another tip from me is start small, i always recomend the rsc gp-9, the maps are easy to learn, you also find out about reversed textures, which is something you have to deal with alot when painting for TS, and it doesnt even have to be a full paint, make a leaser unit and just spray paint over the lettering and make it your own. just have fun with it. thats what iv'e been able to do, is just have fun! 
And one more thing. 
Dont let others boss you around with what you should make. make what you want to make. i've learned that mistake in the past, and its an embarrassing one to make  
 Have fun repainting! Can't wait to see your work!
 
			Da Build: Intel Core I5-9600k, Nvida RTX 2060 Super, 32 GB ram, and way too much storage.