Metal is a common resource in Ark, that is to say common in small quantities. To get it in decent quantities you need to find Metal-rich Rocks. Metal is most commonly harvested with the Stone or Metal Pick, though the fastest option is with Dino Power.
Metal is something you are very likely to need, and in large quantity. It is harvested with either the Stone or Metal Pick and can be found in small quantities in many rocks. It is rare, however, unless you find the rich metal rocks. You may not find much from regular rocks, but River rocks provide a slightly better chance for metal.
Rich Metal rocks are usually quite different in appearance from regular rocks. They have a golden or brass color to them. For the most part, these rocks are only found higher up in the mountains. So far the only other place that I have found rich metal rocks is on the horseshoe-shaped island in the bottom-right corner of the map. This is sometimes refered to as Herbivore Island. These rocks contain large amounts of metal, and seem to always respawn in the same place. So once you find some, you'll be able to return to that location for regular farming.
For rapid metal collection, there is one Dino that does a good job. The Ankylosaurus, or Ankylo, harvests Metal efficiently. It has a decent weight capacity that allows for gathering a decent amount of resources before having to return and unload.