Quests for Classic dungeons come from Wow gold classic outside quest lines, typically in the zones in which the dungeons are located. In some later dungeons you'll find quests indoors, but that practice didn't really begin in earnest before the LFG system was made. This means that in case you want to do dungeons while leveling, you will often complete a zone first, then attempt to discover a group for that zone's dungeon.
If you are brand new to, or jumping back into, the World of Warcraft experience, we hope this WoW Classic starter guide has been handy. Don't forget to buy wow gold have a look at our other guides for more-specific tips on World of Warcraft Classic.