I was a coder who learned to design a decade ago.

Start by opening up your design eye and paying attention to the how and why of everything you see. The design of everyday things is a good book to start opening your design eye.

The material design spec is a good resource for learning some basic design rubric.

Learn the tools that are used to design. Adobe XD is my personal choice. Build tooling inside of XD to work faster, like your own custom sticker sheet. Maybe even get practice at it by designing a design spec by hand. Material design, tailwinds, and bootstrap are all options to start from.