As you are leveling your character in World of Warcraft, you need to go to new places to find new quests and mobs that are worth experience.
For the first few levels in the game, the choice is easy since you are led through the starter areas and the first-out zones.
After that, you will have found a major city and you will start to have more choices about where to go.
Here is a guide for what zone will be appropriate for your level.
The list will be split for Alliance and Horde Alliance Zones: Levels 10-20: Loch Modan (W of Ironforge), Westfall (E of Stormwind), Bloodmyst Isle (N of Exodar) and Darkshore (boatride from Darnasus).
These are natural progressions for the races that come from these cities.
Westfall is the only zone with a real choice since you could also head W of Stormwind - which leads to Redridge (level 15-25).
If you are not following a guide, then I would recommend that you finish as many of the quests in your own zone as you can and then go visit one or more of the other zones on this list.
Bounce between these zones until you are high enough level to finish all of the quests.
This strategy will give you quests that are easier to accomplish and help out your reputation with the nearby home cities.
Levels 15-25: Ashenvale Forest, Duskwood, Redridge, Stonetalon Mountains and Wetlands are good zones for these levels.
Again, if you are not following a guide, jump between these zones to accomplish quests that are your level or less.
Levels 20-30: The Barrens (quests out of Ratchet), Hillsbrad Foothills, Thousand Needles are the zones for 20-30.
Levels 30-40: Alterace Mountains, Arathi Highlands, Desolace, and Stranglethorn Vale are good choices.
In Stranglethorn Vale, focus on the quests out of the rebel camp (in the North) in the earlier levels - Booty Bay generally has higher level quests.
Levels 35-45: Dustwallow Marsh, Badlands, Swamp of Sorrows Levels 40-50: Hinterlands, Feralas, Tanaris Levels 45-55: Blasted Lands, Azshara, Felwood Levels 50-60: Un'goro Crater, Burning Steppes, Western Plaguelands, Eastern Plaguelands, Wintersong, Moonglade, Silithus, Deadwind Pass Levels 60-70: For these levels, both Alliance and Horde should be in the Outlands.
These zones were introduced with The Burning Crusade - which also bumped up the level cap to 70.
These zones are Hellfire Penisula, Zangarmarsh, Terokkar Forest, Nagrand, Blades Edge Mountains, Netherstorm, Shadowmoon Valley Levels 70-80: For these levels, both Alliance and Horde should be in the Northern Isles.
These zones were introduced with the Wrath of the Lich King - which also bumped up the level cap to 80.
These zones are Howling Fjord, Borean Tundra, Dragonblight, Grizzly Hills, Zul'Drak, Sholazar Basin, Crystalsong Forest, Storm Peaks, Icecrown Horde Zones: Levels 10-20: The Barrens (E of Thunder Bluffs and W of Ogrimar), Ghostlands (S of Silvermoon), Silverpine Forest (W of Undercity) are good zones.
If you are not following a guide, then I would recommend that you finish as many of the quests in your own zone as you can and then go visit one or more of the other zones on this list.
Bounce between these zones until you are high enough level to finish all of the quests.
This strategy will give you quests that are easier to accomplish and help out your reputation with the nearby home cities.
Levels 15-25: Ashenvale Forest, Stonetalon Mountains Levels 20-30: Hillsbrad Foothills, Thousand Needles Levels 30-40: Alterac Mountains, Arathi Highlands, Desolace, Stranglethorn Vale Levels 35-45: Badlands, Dustwallow Marsh, Swamp of Sorrows Levels 40-50: Hinterlands, Feralas, Tanaris Levels 45-55: Azshara, Blasted Lands, Felwood Levels 50-60: Un'goro Crater, Burning Steppes, Western Plaguelands, Eastern Plaguelands, Wintersong, Moonglade, Silithus, Deadwind Pass Levels 60-70: For these levels, both Alliance and Horde should be in the Outlands.
These zones were introduced with The Burning Crusade - which also bumped up the level cap to 70.
These zones are Hellfire Penisula, Zangarmarsh, Terokkar Forest, Nagrand, Blades Edge Mountains, Netherstorm, Shadowmoon Valley Levels 70-80: For these levels, both Alliance and Horde should be in the Northern Isles.
These zones were introduced with the Wrath of the Lich King - which also bumped up the level cap to 80.
These zones are Howling Fjord, Borean Tundra, Dragonblight, Grizzly Hills, Zul'Drak, Sholazar Basin, Crystalsong Forest, Storm Peaks, Icecrown