You can start by using plain, semantic HTML and grabbing a classless CSS with a license you like. (Check out this list.)
It’s good enough for a simple app or site, and it’s honestly impressive how good something can look with just this. It’s the “plain t-shirt and jeans” of web design.
If your school has Hackathons, try to do those, ideally with friends. The atmosphere is honestly a bit horrible in my opinion and you may get instant imposter syndrome, but it gives you a project to talk about.