First it begins with the understanding that you will never stop studying and learning. While there are plenty of resources out there to get you started such as books, Youtube, Udemy, Treehouse and others, choose those that align with your goal. If you want to learn design only, then you'll need to focus your efforts on learning the creative side of this field. That means learning about balance and composition, typography, and color theory for starters. If you are more interested in the development side then starting with HTML, JS, and CSS are critical. In addition to that you'll want to learn about accessibility, user experience design, and the ins and outs of available browsers for starters.
Sounds overwhelming right? It gets better. If you want to be good at what you do and well rounded, you'll study both web design and front-end development. That way you'll not only understand how to design something but also why it's designed that way aside from the rookie phrase "Because it looks nice". Ready for the good news? You don't have to do this all in one night. I've been designing and building websites since 1994 and I'm still learning and exploring. There are many avenues you can take in this field and you have the ability to explore all of them. Some you will be great at. Others you will fail at hard. Find that aspect of web design that you love and go! Also, have fun because the world of web design is one gigantic virtual playground.