You obtain medals throughout each level except the first one. These are hidden when you first play the level, but you can still obtain them. After playing through a few areas of the game, you may have obtained your first medal. When you open the World section on the main menu, it will show you the medals you have obtained and this trophy will unlock. I obtained this around 9-1 as that is when I opened the menu. By that point, I had unlocked 10 medals. By the time I finished the game, I had about 27 medals obtained.
You do have to get 70 medals for 2 other trophies if you are going for the platinum.