Here is a Terraria Endgame Guide submitted by Dashspin.
NOTE: This guide might not work with hardcore and mediumcore. This is more a detailed overview than an actual guide. You get a lot of the stuff by going mining, then starting step 3. Good luck! If you have any of your own tips for endgame in Terraria, please post them in the comments below. Thanks for your contributions.
1. Make a base and get and go mining. This should get you good stuff to work with.
2. Get some defense and life (make new armor and find hearts to increase your life.) This’ll help a lot.
3. Get a good bow, some arrows, and maybe an archery potion to beat the Eye. Shurikens may also work. SLEs can be found in gold chests sometimes. If you find a demon altar, you can make one there, too.
4. Make a platform arena. A good example is found in the wiki, but it doesn’t have to have walls, nor does it have be HUGE, just big enough.
5. Wait ’til night and challenge the Eye. Make a a Light’s Bane and sell the rest of the Demonite. Should get you some money. If not, beat it again. The Light’s Bane will get you through corruption.
6. Keep farming the Eye and selling the stuff you get and then store the money for later.
7. Get the dryad to move in and then buy some purification powder. Then head for corruption. Hopefully you have a good enough weapon.
8. Go down a chasm and watch your step. A grappling hook would help. You can also use wooden planks to move down by creating them under you. It's slow, but it's also safe. Also use blocks to protect yourself from the flying mobs, incase yourself and slowly move down, they won't be able to get you.
9. Break a shadow orb or two. Hopefully you get a musket. Then get the Arms Dealer to move in. The musket is a great ranged weapon early on, and ammo is pretty cheap.
10. Gather a bunch of vile mushrooms and make Vile Powder. About 25-30 Vile Mushrooms will be needed. Then break a third shadow orb spam it on the Eater of worlds. Use what you get for a Nightmare Pickaxe and Shadow armor.
11. Now here’s what the money’s for. A MiniShark! You might want to buy a bunch of ammo, too. Grab an ironskin potion and head for the dungeon.
12. Lots of life, defense, and musket balls should let you beat Skelly. Make yourself a worthy arena and challenge Skelly at sunset.
13. Kill the hands first. They’ll get annoying. (Optional, but recommended.)
14. Congrats! You beat Skelly. A meteorite should’ve landed by now, go mine it with Sticky bombs and Dynamite. If you have a gold pickaxe that should help polish off the last bit of it.
15. Make a better weapon. A phaseblade has longer reach and does more damage.
16. You should be ready for the dungeon. Get some golden keys and start opening some chests! There’s good stuff in there. Just look out for the enemies.
17. Get a few extra golden keys and start looking for floating islands. There’s lots of gold and silver ore there, as well as one locked golden chests per island. See the wiki for more on floating islands.
18. Dig to the underworld! A straight shaft is recommended, or if you can’t make it straight, make it MOSTLY straight.
19. The fireblossom stage. Use some water walking potions and look for fireblossoms, or get some ash and put it on the surface. Fireblossoms will randomly bloom on the ash.
20. Did you find a shadow key? You can use water walking potions and start searching for shadow chests. There’s endgame stuff in there. Should help you get to the Hellstone stage.
21. Do you have a meteor hamaxe? Then you can Get your own Hellforge! It works as a furnace and lets you make Hellstone Bars.
22. Now you’ll need some obsidian. Get a lot of it. 5-10 Obsidian Skin potions should be enough to get you a lot of Hellstone.
23. Time to mine the Hellstone! Use obsidian skin potions and get deep under the lava. Make an air pocket so you don’t need to go up. The only enemies you’ll experience are the dreaded Bone Serpents. Just pull out your Muramasa and hold down the attack button until it dies.
24. Make some Hellstone Bars! What you make first is up to you, since they’re all awesome.
25. Head to the underground jungle. Several Healing Potions are recommended for it. Then make some Jungle stuff. You can also look for Shrines.
You’ve pretty much reached endgame now. You can work on your Vanity Items, make an awesome base, or just chill with your friends. Good job.