On your college campus you should put up posters advertising your initiative and potentially asking for people to help you spread the word. You could also contact left-affiliated political organizations on your campus to see if they’re willing to help you propagate your ideas about this. And on here specifically, if you create a website or an app and have links to them, you could make a post on tumblr and ask various leftist bloggers with clout on here to reblog your post so that you can spread the word about it. If you come to me with that type of post I can also reblog it for you. Just be careful about protecting your intellectual property and yourself from any right-wingers who’d try and stop you.
Also for local off-campus leftist groups in your area and hit them up. A group of non-college students is likely to be more varied in terms of both age and background so they may be in more need of a college student with web design skills than a college group would be.