Armor may not seem worth much against the common slime, but you'll be on your hands and knees hunting for the best armor once you come across more intimidating foe. Your Terraria world is set into two halves, Pre-Hardmode and Hardmode. Hardmode comes after you defeat all the bosses, the last being the Wall Of Flesh. After this, new bosses will spawn.
There are tools, armor, and more that you can only get in either Pre-Hardmode or Hardmode respectively. If you're having a hard time surviving, try preparing each move you make by having a goal of what you need to get before taking on each boss.
PRE: Molten
Molten armor is available after you kill the Wall Of Flesh. Technically, you can retrieve it before killing this boss since the ore needed will always be there in the Underworld. This ore is Hellstone and a Nightmare or Deathbringer pickaxe or better is needed to mine it. These are pickaxes made from the ore found in your respective evil biome, Crimson or Corruption. Just make sure it doesn't fall into the lava as you're mining it.
You'll need the Hellstone ore and obsidian to turn the Hellstone into a bar. Collecting obsidian can be tedious. You'll have to drop water onto lava and then mine the then created obsidian and rinse and repeat until you have enough. With this whole set you'll have the bonus of not being able to be set on fire. Molten armor has a total of 25 defense.
POST: Solar Flare
Solar Flare is the best armor in the game in terms of defense alone. The only way to craft it is with Luminite bars that are dropped by the final boss and Solar Fragments which are dropped by the pillars you get before the final boss. Once you have this armor you will be invincible and can re-do all the bosses to find all the uncommon drops. The set bonus for this armor is that you get solar shields and deal extra damage to enemies with it. The total defense is 78.
PRE: Platinum
Platinum armor is the next best armor underneath Molten. It's the counterpart to gold ore spawning in your world. If you have gold instead of platinum, skip this since Gold armor is weak in comparison. Platinum can be mined with any starter pickaxes. It's set bonus is extra defense, giving it a total of 20.
POST: Turtle
Turtle armor is the next best thing to Solar Flare and can be upgraded. You need turtle shells and Chlorophyte bars to craft it. Turtle shells are an uncommon drop by the large turtles in jungle caves so you'll have to do some farming. Chlorophyte is the new Hardmode ore that will spawn after you defeat the Wall Of Flesh and destroy altars in the Crimson or Corruption biome.
It can be upgraded into Beetle armor which has a total defense of 61-73 with a set bonus that increases damage. You need beetle husks for it which are dropped by the Golem boss. Turtle armor has a total defense of 65 and a set bonus for melee and movement speed.
PRE: Shadow
This armor looks the coolest in Pre-Hardmode. Vibrant purple and pink with a speed effect attached. You can get this by crafting it with Demonite bars and Shadow Scales, which are dropped by the Eater Of Worlds boss in the Corruption. Luckily, the Crimson counterpart is exactly the same in stats so you can go either way.
The Crimson armor needs Crimtane bars and Tissue samples which are dropped from the boss Brain Of Cthulhu. Shadow armor increases you're movement speed and Crimson armor increases life regeneration. Both have a total defense of 19.
POST: Squire
The Tavernkeep NPC will start selling a lot of special armor that can only be bought with Defender Medals that are won by taking on the Old One's Army event that you can summon with the Eternia Crystal the Tavernkeep sells. This can all only be done and available in Hardmode. The total defense for the Squire armor this NPC sells is 56 and the set bonus is that it allows you to have one extra sentry summon.
PRE: Jungle
The jungle caves are annoying and tough to traverse through no matter what level you are. But if you can take on the jungle caves, you can take on anything. Jungle armor is crafted by combining jungle spores, stingers, and vines. All can be found in jungle caves.
Spores are glowing orbs you'll find in the cave. Once you break them it will be very dark as they will be the only source of light around them. Stingers are dropped from the bees and spiked slime, and vines are collected by Man Eaters a.k.a those long plants that go through walls to bite you. Jungle armor has a set bonus of decreasing how much you use mana and a total defense of 17.
POST: Red Riding
Red Riding armor is another set that the Tavernkeep sells and that can be bought with Defender Medals. It has has a total defense of 48 and a sentry capacity set bonus plus increases the effectiveness of an Explosive Trap sentry summon. Not to mention it's a really cute and aesthetically pleasing set that also counts as armor.
PRE: Gold
Gold isn't as good as Platinum armor, but if it's all you have it's all you have. It's the best thing you can get underneath Molten, Crimson, and Shadow armor. It's naturally around your world in Pre-Hardmode and can be mined with any pickaxe, really. It has a set bonus of three extra defense, giving it a total of 16.
POST: Frost
You'll need a lot of items to craft Frost armor and look super dope. You need to craft Titanium or Adamantite bars, Hallowed bars, and Frost cores at a next level anvil such as Orichalcum or Mythril. Frost cores can only be dropped from Ice Golems that only spawn when there is a heavy storm in the ice biome. The set bonus for Frost armor is that your attacks cause frost burn and visual and audio effects. The total defense is 43.