Enter your world seed into the interactive seed-map finder on this page to get the exact X/Z coordinates of every Nether Fortress in your world. In-game, head into the Nether and explore until you spot the tall nether-brick bridges and corridors, then bridge or fly outward across new regions, since each region generates at most one fortress.
Nether Fortresses generate only in the Nether and can appear in any of its biomes. In Java Edition they share generation regions with Bastion Remnants, so each region produces one or the other, which means you may need to travel through several regions to find a fortress; in Bedrock Edition both structures can generate in the same area.
Nether Fortresses are the only reliable source of Blazes, whose blaze rods make blaze powder for brewing and eyes of ender. They also spawn Wither Skeletons that drop the skulls needed to summon the Wither, grow Nether Wart for potions, and contain chests with loot such as saddles, golden horse armor, and diamonds.
