I am making a Discord clone, and I have no idea where to even start.

Pretty much what the title says, I know HTML and CSS. I am learning JavaScript and plan on learning it through the project as well. My friend and I are working on this project together, and he is working on the backend. I have taken the responsibility of the frontend. Now I don't know what to do. Where to even start. Can anyone suggest something, like a resource or something that can help me understand how to do this project? I know I have taken something bigger than myself, but that is because I want to push myself. I am passionate about learning, so hard things are not a big problem.