It's Time!

By this time you should have a space gun with a full set of meteorite armor, and around 300 health. I normally do this fight with 400 health but thats just because I find more life crystals before this time. You will want a good amount of crates, I would say roughly 50-100. The main ore/bar we will be looking for is titanium for it's set bonus. 

 To summon the Wall of Flesh grab your voodoo doll and toss it into the lava. This will kill your guide, so make sure you have enough housing for your NPCs. I think around 10-14 houses will be enough at this point.

This fight isn't to hard, it's all the matter of your arena. Once you kill the Wall of Flesh he has a 100% chance of dropping a Pwnhammar that will be used to destroy demon alters which will spawn hard mode ores. 

Crates!

 Do this before fighting the Wall of Flesh!!!

 With the worms hopefully gathered sometime in the beginning, find a pool of water can begin fishing. A fishing pool can be crafted from I think it was 8 crimtain or demonite bars! A crate potion will make it easier to gather crates. It takes 1 moonglow, 1 deathweed, and 1 bottle of water to make this potion.

Do not open any of the crates you fish up, once you begin hard mode, new items will drop from the crates. Hard mode ore is the what we are after!

Demon Scythe

The demon Scythe is a good weapon to be going into hardmode with, it can be a bit tricky to use, so it isn't recommended for bosses, but with it's high amount of damage getting around shouldn't be as hard. 

Slime Staff

Slime have a small chance of dropping a slime staff, which smile from a slime statue can drop. Many people use the smile staff as a way to get money later on in the game so if you get lucky and find one keep it!

Hellstone gear!

You wont really need the armor, but the pickax nonetheless. If you have enough Hellstone bars to craft the imp staff and the hammer/ax make those with whats left. 

Invasions

Goblin invasions are the only invasion you will need to worry about up until after plantara I believe. Fighting a goblin invasion in hardmode will reward you hardmode weapons which many will be good against the mechanical bosses. 

Voodoo Dolls

This can get a bit tricky especially if you use minions but try not to let the guide voodoo doll fall in the lava, this is the item you use to spawn the Wall of Flesh. More than likely a few of them will spawn while the arena is in make.