For general design resources, Hack Design has curated a really good collection of introductory design lessons:
https://hackdesign.org/For specific design questions, I like to search the UX Stack Exchange (https://ux.stackexchange.com/) whenever I'm unsure of a UI design. For example, if I'm designing a pagination component or error messages, there are probably relevant questions and answers related to best practices.
I would also recommend familiarizing yourself with the human-centered design process - nothing beats hands-on user research and prototyping. Identify which parts of your interface are causing the most friction, why, and what users are actually trying to achieve.
A lot of UI can be improved with basic visual design concepts - typography, color, spacing, visual hierarchy, composition.