How do I boot into Internet Recovery on a iMac?
Taken from this support article:


  OS X Recovery includes a built in set of utilities as part of the
  Recovery System. To start your computer from Recovery, restart your
  Mac and hold down the Command key and the R key (Command-R).
  Press and hold these keys until the Apple logo appears.


The recovery feature is limited to OS X Lion, Mountain Lion, Mavericks, Yosemite, and El Capitan.