In the early game, your best bet for metal is the round grey rocks along rivers, this should do you to craft the early tiers of metal items.
When harvesting metal, the Ark Pick is the most effective tool. You can get metal from regular rocks, also, you can buy ARK Survival Evolved Items to get quickly. But the metal pick harvests the most metal when picking it.
The metal output obtained by collecting with tools is also very low. So, players often choose to Buy ARK Items. We can conquer an Ankylosaurus, and the same amount of metal produced by collecting riverside rocks is much higher than our collection.