I'll have to strongly second this one. I was a complete noob with Photoshop before I got into mapping in 2014, and everything I know now I have learnt solely from cartography tutorials. I don't claim to be a pro yet, by any scale, but I have gotten a better grip on the program than I ever though possible. For me personally, the Jezelf tutorials and Tear's Saderan tutorial have been my number one teachers. This site truly is a treasure trove.