06-06-2012
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Join Date: Dec 2008
Last Activity: 28 March 2014, 8:35 AM EDT
Location: Vienna, Austria, Earth
Posts: 5,521
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Thanked 335 Times in 308 Posts
A very simple start: pick the open source software you use most often, register with their bug tracking system, and start testing the newest builds. When something breaks, try to repeat it, and send it in with instructions on how to reproduce, what happens, ...
Over time you might start looking at the code to see why it breaks, or how to improve the documentation.