Take a desktop publishing or a design course, regardless of if it’s in Photoshop or not. The design elements are there.
Then go to Adobe’s site and check out their tutorials, as mentioned or another self-learning website like www.lynda.com
It’s going to be hard to learn design without proper instruction to teach you what to look for, mostly because if you look through the internet, there’s a lot of bad designers giving bad advice on design.
I taught myself Photoshop in high school (Version 5.5) and have been using it since. I’ve taught design professors things about it. They’ve taught me proper color theories and how to make things “pop”. They’ve taught me layout. They’ve taught me what to look for in bad design. I knew a few things, but even regular art classes wont teach you what a design class could. They’re different. I studied art in high school, and was planning on going to school for it - but in college, I took design courses instead of “Drawing 1” and such because I knew that shit, and they’re a different world.
Another really helpful tip that actually got me through most of my early PS days…
Image Mishmash on these very forums. I’d post some work up, ask for critiques, ask for what I could add and what I could take away. If they told me to add something that I didn’t know how to do, I went and looked it up, whether in a Photoshop for Dummies book at the library or something. Today, you’d do that on Google.