UXP for Newbies
Just a couple of things before you start on your journey:
You'll need to know a bit about your host application. In this case, you need to have a passing knowledge of Photoshop. Photoshop itself is a deep, broad program with a long history, but don't get sucked into trying to learn it all. Knowing something about Photoshop layers, selections, and basic tools (such as the pen and type tools) is a good start.
Don't be discouraged. One popular starting point is the Mozilla Developers Network tutorials. But there are different learning styles: some like to read documentation, some prefer YouTube videos, and some like to just dive right in and make mistakes. Find the style that works for you.
An apocryphal quote from Picasso is "good artists borrow, great artists steal" -- which means that, when you get to the point where you're ready to try coding, looking at sample code and putting it in your own projects is usually just fine. Then go through the code at your leisure and make sure you understand what it's doing. And to start you off with some sample code, look here.
Finally, ask questions. There are many helpful people on the Adobe forums, and chances are you'll find answers there.