My approach was to take something I already made or knew well how to do and apply it to the web. That way you aren't struggling with solving an unfamiliar problem but mainly concentrating on how to best implement it with your new tools. For me it was creating an event calendar I had done previously in xBase. I know the logic and it was suitably complex enough to be a good programming exercise as well as a good workout for the layout aspects of web development.

So find something in C that you know well than you can develop on the web, start with that.