I just reached Hard Mode in Terraria after beating the Wall of Flesh, so what do i have to do next? I know that to call the new bosses you need to create an specific item. But regarding the ore grinding and armor sets and weapons, What should i get?
You need to start breaking demon altars with the Pwnhammer. This will spawn the new ores that you need to mine: Cobalt, Mithril, and Adamantite. Each altar smashed will spawn some of the next ore in the list. The first spawns Cobalt, the second Mithril, the third Adamantite, then fourth Cobalt, etc. After each ore is spawned once successive altar smashes will produce less and less of that ore. But it's still worthwhile to smash as many altars as possible. I have heard it suggested that the new ores are more likely to spawn in the underground jungle, but I cannot verify this from my own experience.
Now, I personally like to aim for Excalibur and the Megashark first. That's because you need souls of Might - dropped by Destroyer - to make them. I find him the easiest Hardmode boss to tackle. Because of that, I don't build a lot of hardmode gear (except armor). I only need the swords (I go back and build other stuff later of course). But before I worry about swords or armor, I go for the drills.
It looks something like this:
During this process, you want to farm crystals when you can (if you plan on using guns at all; crystal ammo is amazing) and get as many souls of light/night as possible (can drop from any enemy killed in the underground Hallow/Corruption). Those souls are the gateway item for summoning the new bosses.
Once I can summon the Destroyer, I get a weapon to kill it with. Typically that's a Phasesaber. I make Excalibur (which really speeds up farming him) then I make the Megashark (which I use for the Twins and Skeletron Prime).
My way is not the "best" way, but I find the Destroyer easiest and I rather like those two weapons.Bow lovers can easily substitute repeaters for this process, and you can kill the Destroyer for a Megashark or just try to farm the twins (for the Hallowed Repeater) first. Magic lovers want to forge Crystal Storm.
There are a lot of options. My general advice is not to stop and build a full set of armor/gear at each new metal tier (Cobalt, Mithril, Adamantite). Build as little as you need to progress to the mining of the next tier. If you are dying a lot, then alter tactics (and if need be, do stop and make some better armor).