I know that the crimson/demonic altars do spawn in the crimson and corrupt worlds, but do they spawn in the underground or what? do I have to spend days looking for it and then getting lost because I wandered too far to find it? I need some help.
You will find most of the altars in the corruption or crimson area. Usually, there are caverns or chasms on the surface that will lead to areas with many altars and shadow orbs or crimson hearts (depending on whether your world is corruption or crimson). They are probably about 100-150 blocks down.
You can also find random altars in other biomes besides corruption or crimson, but they not very common. All the altars will all be at roughly the same depth, and aren't that far past the surface.