Hi Jialu, glad you want to get involved! Adding tor-dev@ in case the wider community wants to chime in with ideas. The first step of getting involved is to figure out what strikes your fancy and that, obviously, nobody but you can decide. ;)
Since you mentioned Vidalia one thing you might want to consider is if you like to work on projects alone or collaborate with others. I include the 'Activity' column on...
https://www.torproject.org/getinvolved/volunteer.html.en#Projects
... so you can easily determine for any given project if you would be working with others or adopting a space all your own. Vidalia is firmly in the later camp. Nobody has been actively working on Vidalia for almost a year since chiiph departed.
If GUIs strike your fancy then a project you might want to consider is a new stem-based UI. This is an idea we've been batting around for a while now but has yet to get off the ground. Kamran created a python GTK UI a few years back as part of GSoC...
http://inspirated.com/2011/07/17/summer-of-code-progress-garm-0-1
This piggybacked on arm, but since his work our controller libraries have greatly improved to a point where we should be able to make a solid, new Tor GUI without that sort of kludge. Like Vidalia, though, nobody else is working in this space so it would involve taking the lead on a new project of your own.
If any of the projects on the volunteer table strike your fancy then my best suggestion is to dive in and get involved. Try to fix a minor issue or two then reach out to the maintainer mentioned on the table.
Cheers! -Damian
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Damian Johnson