The first Open-Source contribution I remember happened on December 2005. It happened on Gaim, which is now known as Pidgin.
It was not a big contribution: I did not like the arrows shown when moving discussion tabs. Those arrows where plain red. I created new ones (using The Gimp), with a gradient. It did not take a long time to make them, but it was definitively an improvement.
Here it is:
It was quickly merged (< 2 hours). And it was important for me, because at that moment I understood that I could improve the tools I was using, and that everyone would get those improvements. That was the beginning of 12 years of Open Source contributions.