So, if you haven't enabled Hardmode yet, you should prepare yourself. Here's a list of things you should have made:

1. Full Molten armor.
2. A Night's Edge (Or another weapon with high DPS)
3. A Reaver Shark or Molten pick
4. The Amazon (YoYo)

Now, what you need to do before entering HM:

1. Purify as much Crimson/Corruption as possible.
2. Contain it so that it doesn't spread.
3. Farm up some jungle spores, in case your jungle gets hallowed/corrupted.

Now, you should be ready to enter HM.

So, if you have just enabled Hardmode, the first thing you should do, is grab your Pwnhammer that you got from the Wall of Flesh. If you deleted it, you're dumb ._.

Now, go to your Crimson/Corruption, and smash 3 altars. You can smash more, but that will pollute your world. A lot.

Smashing the altars generates new types of ore in your world:

1. Cobalt/Palladium
2. Mythril/Orichalcum
3. Adamantite/Titanium

So, don't go mining that new ore yet, you might be killed while doing so. You need a decent weapon first.

Go to your snow biome, and make a hole, hide in it, and leave one block so you can stick out your Amazon yoyo.

Now, wait for enemies to spawn.

When a lot of them have spawned, start killing them. Every thing killed in the snow biome in Hardmode has a 1/80 (1.25%) of dropping the Amarok. The Amarok is a pretty decent yoyo, it can help you in early Hardmode. It does 43 melee damage and inflicts the Frostburn debuff.
Now you can go mining for them new ores.

So, now you can go mining for those new ores!

First, you need to mine Cobalt/Palladium. You can only mine it using a Reaver Shark or a Molten Pickaxe, so make one if you don't have it already.

Once you have a lot of Cobalt/Palladium, make yourself a set of armor (I reccommend making the melee helmet, since it gives you way more protection than the other ones) and a pickaxe. You need it to mine Mythril/Orichalcum.

Note that making things from Mythril/Orichalcum and above requires a Mythil/Orichalcum anvil, those are made from 12 bars.

I reccommend that you skip Mythril/Orichalcum armor, and just make a pickaxe so you can mine Titanium/Adamantite.

If you wanna find a lot of Titanium/Adamantite, you can find at at the magma level, or at the roof of the Underworld.

Now, once you have mined tons of Titanium/Adamantite, it's time to make a set of armor. Again, I reccommend making the melee headpiece, as you don't lose a lot, and it offers you way more defense.

Now, it's time to hunt for something special.

Now, go to your Hallow biome, and go underground, until you reach the Underground Hallow. Killing enemies here can drop Souls of Light. Once you get 15 of them, go to a crafting table, and make a Key of Light. Make a chest, and place it in there, make sure the chest is empty tho. A mini boss will spawn, called a Hallowed Mimic. They are really strong, so I reccommend you avoid them as much as you can. Once you kill it, take a look at what it drops. Everything is useful, but we are looking for the Daedalus Stormbow. If it didn't drop it, repeat this process once again until you have it.


Now, with the bow, you need Holy Arrows. You can craft these with 200 wooden arrows, 3 pixie dust, and a unicorn horn.

Now, we are ready to take on the Destroyer.

Now, it's time for your very first HM boss fight.

You need to craft the Mechanical Worm, which will spawn the Destroyer.
It is crafted at a Mythril/Orichalcum anvil, and requires 6 Souls of Night, 5 Iron/Lead bars, and 6 rotten chunks.

Make an arena for him.

A typical arena is made out of wooden platforms, but it's personal preference.

Wait until night, and summon him. This is where the Daedalus Stormbow comes in handy. Once he is here, keep spamming arrows with the Stormbow, this should kill him pretty quickly. Bare in mind, that he spawns probes which shoot you, and they can be very annoying.

Once you have killed him, you know what to do next.

Thanks for reading! If you see any mistakes, or things I could add, please note in the comments!
Hope this helped you! :D