Terraria – How to enable Hardmode

The new ores are generated when you destroy Demon Altars in Corruption or Crimson Altars in the Crimson, not when you kill the Wall of Flesh. Killing the WoF does enable the destruction of Demon Altars, though. Upon defeating the WoF you receive an item called the Pwnhammer, this is what you use to destroy the Demon/Crimson Altars.

Note that they can only be destroyed in any world where the WoF has been beaten, so you can't take a Pwnhammer to a new world and start bashing Demon/Crimson Altars.

3 things happen when you go to hard more:

1. stronger enemies and bosses appear.
2. Corruption/crimson spreads quicker
3. Hallow biomes appear and spread.

To prep for stronger enemies/bosses, you should make sure you have top tier pre-hardmode weapons/armor/accessories and 400 max health. ie: Molten armor and a strong weapon like nights edge.

Getting a surface mushroom biome with a house for the Truffle NPC and then buying a mushroom spear as soon as he moves in after hardmode will make surviving much easier. The spear has decent knockback, good damage, and can hit enemies through walls.

You should get a Molten pickaxe to make Colbalt/Pallidium mining possible early on. quickly breaking Demon Alters and raising yourself to at least oricalcum armor quickly will make the common enemies much more bearable.

If you care about the lose of your natural biomes then the spread of corruption/hallow is a problem. You cannot control where it spawns but you can harvest the biomes blocks and create artificial biomes later on. However, I think waiting until you can get the Steampunker NPC and can buy the Clentaminator to remove hallow/corruption is a better use of time.