Netherite is currently the highest tier of weapons, tools and armor you can get in Minecraft. However, it's also the most difficult to obtain. All netherite items, with the exception of lodestones, are lava proof.
To upgrade to netherite, you will need to use a smithing table, with is crafted with a square of 4 planks and 2 iron ingots on top. Only diamond equipment can be upgraded, so that's required as well. Along with a netherite ingot, you will need a netherite upgrade template.
Netherite upgrade templates can be found in bastion remnants. Bastion remnants are divided into 4 types of structures, 3 of which have unique loot chests, alongside the generic loot chest all 4 share. Netherite upgrade templates might occasionally be found in bridge chests, hoglin stable chests, or generic chests, but they are guaranteed to be found in a treasure chest of a treasure bastion. Once obtained, they can be duplicated like armor trims, with the template at the top, material at the center (in this case it's netherrack), and 7 diamonds.
Netherite ingots can sometimes be found in the treasure chest of a treasure bastion.
Alternatively, they can be crafted with the following ingredients:
Netherite scraps can sometimes be found in bastion treasure chests, or rarely in bastion stable chests or generic bastion chests. They are smelted from ancient debris. I don't recommend smelting ancient debris unless you have enough for an ingot, since that crafting recipe is their only use, while ancient debris on its own has other applications.
Ancient debris can sometimes be found in bastion treasure chests, or uncommonly in bastion stable chests or generic bastion chests. Ancient debris is an ore in the Nether that can only be mined with a diamond or netherite pickaxe. It generates most at y=16, quickly tapering off above and below to y=7 and y=25. It never generates exposed to air, or above y=119. Ancient debris has a high blast resistance, so explosives such as TNT or beds can be used to search for it.
Unlike other blast resistant blocks, except for netherite blocks and falling anvils, ancient debris can be moved with pistons, making it useful in certain redstone contraptions.
Bear in mind that upgrading always costs a netherite ingot and upgrade template, so generally it's more worth it to upgrade something that cost more diamonds to craft. Don't worry about enchanting yet, since netherite has higher enchantability than diamond, and upgrading preserves the enchantments. All netherite items have higher durability than their diamond counterparts.
The Diamond Pickaxe should be upgraded to netherite first, since that's the tool many players use the most. If you have pickaxes already enchanted with fortune and silk touch each, I'd prefer upgrading the silk touch one first, since mending gives back durability when mining ores with fortune.
Java: The Diamond Axe should be the next tool upgraded, because it deals the most damage out of any tool, and more than the sword in Java Edition. Additionally, it's the most cost-effective tool to upgrade, alongside the pickaxe, at 3 diamonds per netherite ingot. As with the pickaxe, upgrade the silk touch axe first. If your axe has a weapon enchantment, upgrade your sharpness axe first, unless you plan on fighting the Wither, in which case you can upgrade the smite axe first.
Bedrock: The Diamond Sword should be upgraded after the pickaxe.
If you got 4 more netherite ingots, after doing the upgrades previously mentioned, upgrade your Diamond Armor. It's better to wait until you can upgrade all of it, since netherite armor doesn't add more armor points, but adds 4 more toughness points to diamond armor's 8 toughness, for a total of 12 toughness. Each netherite armor piece gives 10% knockback resistance, for a total of 40% knockback resistance.
Java: Upgrade your Diamond Sword next for higher damage per second.
Bedrock: Upgrade your Diamond Axe next, since it's the most cost-effective tool to upgrade, alongside the pickaxe, at 3 diamonds per netherite ingot.
Next, upgrade your Diamond Hoe which on Java edition gives you the Serious Dedication Advancement.
The last tool to upgrade is your Diamond Shovel, since it's the least cost-effective tool to upgrade at only 1 diamond per netherite ingot.
Blocks of netherite are crafted with 9 netherite ingots, and you can get the 9 netherite ingots back from a block, making it a resource block. Like ancient debris, netherite blocks have a high blast resistance and can be moved by pistons. Blocks of netherite can be used to power a beacon, along with blocks of diamond, emerald, gold, and iron.
Netherite ingots can be used as an ingredient in armor trims, having a dark grey to black color pallete. You can't get the ingot back, but it's not a total waste, since it's color appears on the armor.
Lodestones are guaranteed to be found in bridge bastion chests. If that's not enough lodestones for you, they can be crafted with 8 chiseled stone bricks with a netherite ingot in a middle. Using a compass on a lodestone makes it so the compass points to the lodestone. This can be helpful in the Overworld, and especially the Nether or End, where maps can't be used. I guess if you have big projects in the Nether or End, lodestone compasses can be useful to have?
Netherite ingots can be used to repair tools, weapons or armor in an anvil. To that I say to apply mending or upgrade a new netherite item. If the item is too expensive to apply mending to, or if you're low on diamonds but have plenty of netherite ingots, then I guess it's fine, except you can reset the enchantment cost with a grindstone.
Perhaps the most egregious use of netherite ingots is in setting the beacon power, since diamonds, emeralds, gold ingots, or iron ingots could be used instead.