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).
First off, start by smashing demon altars and leaving only one behind for crafting purposes; then mine the other new ores that come from destroying demon altars (cobalt,mythril,adamantide and their other related type) to make good armour and weapons.
Then after fight one of the mechanical bosses to make the jungle spawn planteras buds; these can be destroyed to summon Plantera.
After defeating plantera you will receive the temple key which can be used to open the lihzard temple door inside The lihzard temple you will find a lihzard power cell which can be used summon the golem.
Once you've managed to beat the Golem, congratulations, you've covered a major milestone and it's time to try to take on some other bosses as well as work on your base and maybe make an armoury to keep your armour on mannequins, a museum for statues and frames,a variety of gardens, and so on.
Iff you think you've got what it takes, try the pumpkin moon or frost moon, and be sure to ready up a load of traps and defenses for your base; possibly even safehouse that you can hide in when the going gets tough.
If you get bored afterwards you can try a replay with a different style for self-imposed challenges.
Well start mining and get new ores and gear up with new ores ( gear up with weapons as well ) than we'll fight some bosses and off you go basically