Oh, These Baubles?
Medals unlock if you have settings on to see them or not to see them(To turn your medals on you must do under options while setting up your mission, it will be called Scoring and Time HUD). For this achievement you need 6000 campaign or Matchmaking medals unlocked in total.
While playing you will naturally unlock medals but keep in mind while playing career you want to kill as many enemies as fast as you can, dying the least amount you can. Doing this you will get multi kill medals and killing spree medals.
Playing on Easy difficulty in the campaign will be faster then playing legendary because you don't gain any extra medals for doing Legendary. Easy will make you die less and kill more without much effort.
To view all medals and the ones you currently have, press the Start button anytime while not in a current match or mission (Anywhere on the menu) go to Career, then hit cn_RB to see your medals. Once in medals you can use your cn_LB and cn_RB to scroll medals from Matchmaking, Custom, Campaign, and Playlists. Below the medal icon it tells if its in Halo 1, 2, 2 anniversary, 3 or 4.
If you play splitscreen and are the 2nd player (probably 3rd and 4th too) you will not get medals towards this achievement. (Confirmed by Kooky Cow 32
If you plan on going for the other achievements 10,000 medals is nothing and you should not go out of your way, I haven't even played the Halo 3 campaign yet and am halfway to 10,000. I just thought it was worth noting.