Writing CSS is easy, anyone with basic knowledge of CSS can make something look good. The hard part is to write maintainable and scalable CSS.
Here are few things I would recommend to a beginner:
Naming conventions and architectures help you structure your CSS and markup in a logical way. Read about the different conventions and learn about the “why” and the “how” of them, this will help you and your team to stay on the same page.
BEM, SMACSS, ITCSS are just a few to name.
Organize your CSS. Use a preprocessor, split your CSS into parts, organize responsive CSS. Proper organization is the key to maintainable CSS.
There are many UI frameworks out there, don't follow anything blindly. Learn from them, replicate the good things and drop the bad parts.
If you do decide on using a framework then make an informed decision. Ensure that the framework doesn't become a dead weight to your project.
You only become good at something when you practice. Declarative nature of CSS makes it hard to grasp at the start. Keep on practicing, ask and learn from your fellow developers.