After announcing a limited technical preview (limited as per invitation), and following a more private beta program before, Microsoft has announced today that Office 2016 is now in public beta, meaning everyone can access the beta build and start testing it 
Satya Nadella has announced it during his keynote at the Convergence Conference at Atlanta http://www.microsoft.com/en/convergence/atlanta15/#fbid=yZlbL-Hz15r
More details about what to expect from Office 2016 are here http://blogs.office.com/2015/03/16/announcing-the-office-2016-it-pro-and-developer-preview/
You can get the beta builds here http://go.microsoft.com/fwlink/?LinkID=528801
You can use the dedicated forum here http://community.office365.com/en-us/office_2016_preview/f/default.aspx
Enjoy