For long, I was afraid to contribute to any project because I wouldn't understand the whole structure, and I didn't know where to ask.
Then I started using Qt and submitted my first bug report. The bug was fixed in like, 10 hours.
Wow.
Then I submitted another one, but this time I looked at the source to understand what was wrong and suggested a way to fix it.
Then I actually sent a patch.
Then I signed up to the mailing-list, and started talking to people on IRC.
But my real epiphany was with KDE. When I saw my first commit into KDE's trunk, even though it was a tiny one line bugfix, it felt so great. I started wondering : why didn't I do this before?
So, if you've never contributed to OSS, go ahead. Pick a project of interest. Get yourself on the mailing list and on the IRC channel. Ask questions. Find stuff that needs to get fixed. Help others. And get those bugs fixed.