Minecraft wiki entry for Defeating_a_Nether_fortress, associated text: When traversing the Nether to search, especially if you have found golden apples or have enchanted your armor, simply wear a suit of unenchanted iron armor while you explore. There are many hostile mobs in the Nether and you might never be able to retrieve your items if you die. If you fall into a lava sea, all your items are pretty much guaranteed to be incinerated.
You are looking for a large structure that stands out from the ordinary red (or grey or blue, depending on your biome). It's massive and made out of dark red bricks. How box-looking some parts are should serve as a peculiar feature. Some other things you might stumble on:
You have found a small broken portal, perhaps with some stairs or slabs of blackstone. These are ruined portals, and contain a loot chest and blocks of gold. Be careful, as opening the chest and mining the blocks of gold will provoke nearby piglins.
You have found a large towering black structure, smaller than a nether fortress but imposing nevertheless. This is a bastion remnant, which contains many blocks of gold and loot chests (maybe even a magma cube spawner if you're lucky) but if you're playing 1.16.2 and up these are best saved for late game, and even in 1.16-1.16.1 they are best saved for after you conquer the nether fortress and have potions (see below).
Got a saddle? Great. Found a biome of blue with blue trees and endermen? Even better. Harvest some warped fungus from the warped forest biome, make warped fungus on a stick, and now you can ride the small red mobs (called striders) that spawn frequently in lava seas. These are incredibly useful for traversing Nether seas, especially large ones where you might not be able to see the other side. You can also simply sit on one and snipe at nearby mobs that might try to kill you.
If you are annoyed with trying to find a nether fortress and have cheats turned on, you can run the command /locate fortress which will return the coordinates of the nearest nether fortress. If you have the seed of your world you can enter it into a number of online apps which will show you where all the nether fortresses in your world are.
Turning your render distance up will allow you to see farther, but it will also increase the lag on your computer, a possibly fatal factor if you are ambushed. Moreover, because of the Nether fog, you won't be able to see farther than about 12 chunks, so a higher render distance won't help you from that point upward.