There's a total of 120 Relic scattered about the islands, but for this achievement you'll only need 60. The Relics come in 4 forms and each form varies on its location and difficulty to find. Some will be out at sea and/or under water, while others will be up in the hills.
As you play through the game and unlock new Radio Towers, you'll notice an Urn like icon on you map. These are the Relics. You can also buy Treasure Maps from stores to help you unlock their locations. Now simply locate and grab 60 of them for the achievement to unlock.