Main quests and side quests
(i.e. usually, if you do a quest and a human gives you the quest, you'll get Stormwind reputation, if it's a Night Elf you'll get Darnassus reputation, etc.) the reputation lets you buy their mounts. (For example, as a Gnome, normally the mount I would get by default is a Mechanostrider. But if I work really hard for Darnassus and get what's called 'Exalted' status, meaning I got a WHOLE lot of good rep with them, I would get to buy a Nightsaber mount!)
There really are no 'main quests' in WoW, but there is a built-in quest tracker that can help you see where you need to go for quests. If you hit the yellow ! in your minibar (that has your picture and all the other stuff in it too,) you can pull up the list of all the quests you've got and tell it to 'track' them. You'll then see exactly where to go on your in-game map!
In fact, you really have no choice; if you follow the quest chains to where they lead you, you will eventually leave human territory behind and go to more exotic lands. As a hero, you are expected to aid all the allies that need your help, no matter their race. So if you meet a group of Gilnean worgen who could use your help, go ahead and help them! They may give you Gilneas rep, which in turn can help you build up towards buying things from their quartermaster (including their cloaks, bag, and mounts!). Likewise with other friendly factions.
There are only a few factions where getting rep with one harms your rep with another, but you have to make a conscious choice to help one over the other.
If you don't want to quest in a particular zone anymore, you can check the bulletin boards in the major capital cities and see if they give you a "breadcrumb" quest to start you in a different zone. There is no real "main quest" or storyline to follow. You can quest wherever you want. :)
You can also check your map and hover over the zones to see what levels each zone is appropriate for. You can also look at this list: http://www.wowpedia.org/Zones_by_level
However you don't have to be 90 to run dungeons. Read up on them a bit before queuing into the Dungeon Finder (that eyeball icon in the bottom menu bar, available at level 15) and you'll be grouped with other players in a dungeon that's proper for your level.
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