I find this book really helpful: https://www.refactoringui.com/

Along with reading Nielsen Norman articles on best practices and such.

For UI and widgets, I personally like MUI and find it better the Shadcn, but whatever, just start with something usable and polished enough, even if it's not the prettiest, and improve it later when you can hire a designer.

You might also be able to find some relative cheap freelance design just to get a basic look and feel and maybe a logo.