What I usually do is:

If undead: Level through lv 5 or 6 and then move to orc start area:

Do all quests there apart: collection quests and from there move to troll village pick all quests, then move to razor hill: pick all quests there, and do all the quests/ don't forget to go up the tower when you kill benedict and loot the quests from chest there. When you complete all the quests there and you're level 8/9 get the quests to meet near the burning blade cave north east and farm there until you're level 10/11.

From there move to Orgrimmar, pick all quests, move to Crossroads,get the quest to Ratchest, pick the quests there. Do the basic quests that you've received. From there, move to Undercity quickly, pick the quest for Rfc, move to Sepulchur. Do the basic kill quest there and once you're finished there you should be around 12/13.

Hs to Orgrimmar (don't forget to set your HS there) and move to Crossroads and do the other set of lv 13 quests there (mostly the centaurs bosses and the lake discovery). Once you reach lv 14, I'd recommend waiting in Ogrimmar for buff.

The Rfc farm will take you to the lv 19 (you should stay there until that level).

From Rfc farm, you should move to Crossroads, do the quests for Wc and complete the Wc quests there and farm Wc until lv 22.

From there move to Bfd or do the quests.

I'll posts later this week detailed guide for quests and dungeon mix up to 50.