Just realized that my first answer might not be the most helpful :-P. If you're still interested in becoming better at design, it's all about learning-by-doing. Much like development.

I spend a lot of time on https://dribbble.com/ when I initially started designing and that's often where I go to find inspiration these days as well.

Besides that, I'd suggest to doing as many design-related projects as possible. They don't have to be big. They just need to force you to go through the process of going from thought to an actual design. A lot of people practice design by re-designing already existing apps.

The tool of the trade nowadays, when it comes to websites and apps at least, is https://www.sketch.com/, which I can highly recommend.

Hope that helped a bit more.

That's a tough one. One suggestion would be "don't". Most companies want either one or the other. I'm not a brilliant developer or a brilliant designer, which makes it hard for me to find my role in bigger companies.

On the other hand I can make things like https://online-solitaire.com/ and https://habitualapp.io/ as a one-man operation.