Uncharted: Drake's Fortune

The first treasure available can be found in Chapter II, The Search for El Dorado. From the start of the chapter, follow river on the left side until you reach a large rock you have to climb over. Jump down on the other side and the treasure should be at Nathan's feet.

Gamefaqs user YYZed has put together a comprehensive guide detailing all the locations:
http://www.gamefaqs.com/ps3/932984-uncharted-drakes-fortune/...

For reference, here is a video from youtube showing all 60 treasure locations (credit to youtube user dragoszontara).