Poses can be changed in the customisation menu and alter what your Spartan looks like in the pre-game lobby.
If you want to knock this achievement out pretty quickly, one of the first poses you unlock is called "Heroic" which has your Spartan standing side on. It looks pretty crap, but that's just me being picky.
It'll appear as an option several ranks before you can actually use it (because 343i love to tease us), and you unlock it by ranking up to SR13. Obtaining that rank shouldn't take too long; it took me around ten good games, but obviously it'll take different lengths of time to get to depending on the player.
Once you've reached that rank, you should get a notification that it has been unlocked after the match. Then, head into the customisation menu, find and select the option to change your pose, scroll down to "Heroic" and the achievement should pop once you've applied it.
Obviously, you don't have to use the "Heroic" pose, there are loads to unlock further down the line, but it is probably the fastest one to gain access to.