The best place to start is searching for JIRAs with the "newbie" label.
Here are a couple YARN ones that I'd be happy to help you with:
On Sat, Oct 19, 2013 at 3:34 PM, Amr Shahin <[EMAIL PROTECTED]> wrote:
> I'm interested in contributing to hadoop but i'm not sure where to start. I
> was wondering if someone would be interested in being my mentor for the
> first couple of contributions maybe