How to Get Into Hardmode on Terraria

Get some equipment. If you've destroyed the Eater of Worlds and have a lot of Shadow Scales, then make yourself some Shadow Armor. It might take a few times killing EoW (Eater of Worlds) to get enough Demonite or Shadow Scales. Once you have enough for a full set, go ahead and make each armor piece. This is typically the best pre-Hardmode armor in the game.

Go down. Once you have the armor and preferably a ranged weapon (such as Sharanga, which is made from two Molten Fury's, which are made from a considerable number of Hellstone Bars), you need to go down to the underworld. It's easy--just dig straight down until you get there.

Build an arena. When you get down (hopefully you brought a lot of solid blocks), build an arena--nothing special, just a giant bridge from one side of the world to the other. Kill a voodoo demon, and it will drop a voodoo doll. Once finished with the arena, look at the voodoo doll's direction. The Wall of Flesh (the final pre-Hardmode boss) will spawn close to the end of the world that the doll is facing. Go all the way to that side, then move back maybe thirty blocks.

Spawn him. If you think you are ready, then either throw the doll into lava or just tap it, depending on which update you own. One of those methods will work.

Destroy! The Wall of Flesh will then spawn fairly close to you, so attack it with your ranged weapon a few times and then run backward. It spawns from its mouth dozens of minions called 'The Hungry' and once a certain number of them are killed, they become less abundant and are replaced with leeches.

Keep going. It's a good idea to stay fairly far from him, distant by at least 20 blocks. He has three main body parts: two eyes, and a mouth between the eyes. Just shoot him and shoot him and try not to burn in lava. Hitting the body parts deals damage not just to that part, but the whole Wall.