Alliance
Travel from Darnassus to Ironforge or Stormwind
This is how you get form the Night Elf lands to the Human or Dwarven/Gnome lands:
There is a portal behind the bank in Darnassus that takes you to the Night Elf port city. From there you can either take a boat or a gryphon to Auberdine which is in the Darkshore zone. From Darkshore you take a boat (the southern boat) to Menethil Bay in the Wetlands zone. You must then run on foot from Menethil Bay to Ironforge in Dun Morogh. Ironforge is the Dwarven capital city. You can catch a free ride on the monorail in Tinkertown that will take you from Ironforge to Stormwind, which is the Human capital city. Tinkertown is a district of Ironforge.
Wetlands is a level 20+ zone, so be careful and don't stray from the path. Loch Modan is a level 10+ zone that isn't too dangerous, especially if you stay on the path. Dun Morogh is the starting zone for Dwarves and Gnomes, so it will be around the same difficulty as the Night Elf starting zone with mobs averaging around level 7 or 8.
Behold the power of Microsoft Paint!
World Map Diagram Wetlands Loch Modan Dun Morogh
Horde
The Zepplin will take you between Orgrimmar and Undercity.
Orgrimmar: Just East of the Orgrimmar gates is the Zepplin tower. Climb to the top, and get aboard the zepplin that arrives on the SOUTHERN side of the tower. (The north side takes you to the jungle.)
Undercity: The Undercity Zepplin tower is just NorthWest of the city. There is only one Zepplin stop here.
(someone please fill in the other (jungle) zep flight path, I haven't used it yet. --Klocwerk)