[erenemre]

When thinking about getting better at design, do not think making things pretty.

Instead, think how you can solve problems, how you can make your users' life easier. And this actually begins with a lot of thinking.

I cannot recommend you any specific sources but most of the people learn design by trying. If you want to improve your visual skills, look at other designers' works. Try to copy them (do not steal :), read tutorials.