When you open your map you will see quite a few radio tower icons. You will want to clear them all eventually so you can fully open up the map. You must climb each radio tower to the top. None are too difficult (they are just like climbing a view point in Assassin's Creed).
You can easily unlock the trophy very early in the game. You can skip playing the main missions and go exploring. Note that for each radio tower you activate, not only will you open up the map but you will get free weapons and discounts in the weapon shops.