Terraria: What all should I have done before entering Hardmode? (Weapons, resources gathered, etc)?

You should farm up enough hellstone to make the full molten gear (at least) along with the molten weapons and tools, you'll need them for hard mode ores. You should also fortify your defenses on your base, basically you'd want to have some lava pits (make them shallow so you can get loot still) and reinforce your doors (I suggest using active stones as a lock for the outside if possible, else carve out a niche into the ground to have mobs keep jumping, as certain mobs can break down doors easily in this mode.

You should also protect any biome you want to keep by cutting around it and replacing the cutting with mud. 3 tiles wide and 4 tiles high (for digging under the biome).