Finding a Fortress
This is relatively easy, as many portals spawn you near, or even in Nether Forts. If you can't see one nearby, go straight in any direction, and you're bound to find one.
Living In a Fortress
If you've mastered the Nether, you can go in and start making a nether fortress more inhabitable. If you can have control of a Nether Fortress on a multiplayer server, you can control who enters the area, and also control who gets blaze rods (as blazes only spawn in Nether Fortresses). With the addition of Nether Quartz, the one who controls a fort can also control who gets Nether Quartz from the area.
First, you should repair any bridges, as this can prevent falling deaths in the nether, though it gives mobs more access to the fortress.
In many places, lava may be leaking in, so it is advisable to close off the area.
Almost all forts have a single roofed, large building, which you can use as the main building, where you can store all the necessities. You should put doors on the building, to prevent destructive mobs from entering.
A bed is unnecessary, though all crafting benches are needed, and an ender chest might be useful (if one leaves the Nether in a hurry, they can get their items via another chest).
You should disable blaze spawners, so you can use them for mob grinding purposes later on, but also prevent blazes from overunning the fortress, as they are extremely hazardous.
Some parts of a fortress have been filled in with netherrack and it can be a pain to dig it out. TNT is useful, though this risks damaging the fort, creating more hassle for the user.
If one finds a nearby fort that is not attached to the first one, Nether Brick can be looted to repair the fortress if in survival.
Killing the various mobs that spawn there is tricky. Blazes, Zombie Pigmen, and Ghasts pose a threat, though Wither Skeletons are a minor annoyance. Mobs spawn at a faster rate in a Nether Fortress, and trying to kill all the zombie pigmen is impractical, so it is best to just leave them alone, as a large pack of pigmen could knock you off into the lava ocean.
If imported farming does not work, you should use the naturally growing mushrooms to make soup, so you have a bit to eat, though it is preferable to make better food, such as bread, with imported dirt and wheat seeds.
With the addition of Nether Quartz Ore in the 1.5 Redstone Update, exploration of the Nether was increased, and as a result, use of the Nether Fortresses also increased, due to users wanting to get quartz for the Daylight Detector. It is unknown if any new Nether mechanics will be added, though, as the next known update after Redstone is focused on mods, and nothing new for the Nether is on the Mojang Planned list.
Building a Nether Fortress in the Overworld
Note: This is showing if it was 5 blocks long