With the Pwnhammer, you can finally break them. This means at least going underground in the Corruption/Crimson though it's not necessary to go to deep and have to deal with stronger enemies. You can break more later, but may want to save one in case you have trouble crafting a Terra Blade later.

smash shadow orbs, the more you smash the more ore you get Wrong. You have to break Demon Altars with the Pwnhammer to get Cobalt, Myhril and Adamantite to spawn.

I recommend that you go down the the lower parts of the cavern layer, drink a spelunker potion and dig horizontally. This is probably the fastest way to mine the hardmode ores. Or, you can just start fishing up crates and opening them. If opened in hardmode, they sometimes contain the hardmode ores (wooden crates only) and bars (all crate types).