I've seen amazing stuff built by students - the React Create App was created by a student !
https://mxstbr.blog/2016/12/a-dream-come-true/
Incredibly inspiring story, and it shows you don't have to build something huge or amazing to help !
Closer to home, a coworker wrote some simple Javascript helpers for the same reason, he didn't find anything that helped him online, and it's amazing to see them everywhere.
The Javascript environment in particular is ever-changing and can use some help :) , and there's new stuff every day where you can contribute.
OTOH, I wouldn't go to contribute directly to the Linux kernel or any big, mature project where serious experience is needed (and even then, they do need help although they might direct you to help with the docs or stuff like that).