I know, you're thinking "wait a minute, I'm already a human, aren't I? Isn't roleplaying a human in WoW just like being a human in real life -- plus some sword and sorcery, minus some boring office jobs and unpleasant bodily functions?" The answer is no, it's not so simple -- there's a bit of history and culture at work in Azerothian human society that all roleplayers of human characters need to be aware of. Otherwise, it's easy to fall into the trap of inconsistency with the Warcraft lore and the roleplaying that everyone else is trying to do within it.
Suppose for example that you say "Hi! My name is Walter and I was raised on a farm. Now I've come to Stormwind to have adventures and become a hero!" You may find the never-seen-danger-before style of new hero interesting to roleplay, but it would be very unlikely to find such a human in the actual Warcraft lore: ever since the orcs first came through the portal 30 years or so prior to the setting of our game, every human nation has suffered terribly as the human race barely survived 3 huge waves of devastating warfare, with some whole nations of humans completely wiped out. No human growing up in that time would have been untouched by the conflict -- and if you want to roleplay a human, you ought to know about it.
Similar issues exist for all the available player races in WoW; there are certain details about your race's history that you need to know in order to roleplay well. So today we will provide you with the basic knowledge you need to be a human. We'll leave the in-depth lore to other columns, though -- today is just a basic roleplayer's primer on one race, with other races to follow in the future.
In the beginning...
If the character you have in mind is like a human of the historical European dark ages, you're probably not too far off the mark, since those were times of deep hardship, bloody warfare and prevailing disease for that particular portion of humanity on Earth, and much of the culture from that time has been borrowed in the creation of humans in Warcraft: big cathedrals built by menial peasants, feudal civilization ruled by privileged nobles, and an order of religious warriors that guides and protects the common people.
A major difference between any Earth humans and those of Azeroth, however, is that humans on Azeroth have always been surrounded by strange and magical creatures with whom they must cooperate or compete for survival. The history of humans begins 3000 years ago with their teaming up with the high elves in wars against the trolls, and learning from the elves the arts of magic and civilization. Their interaction with the long-lived elves must have led them to feel inferior, both in terms of their lifespan and their knowledge, while their warfare with the trolls led them to feel intense hatred for savageness and monstrosity, as well as the need to dominate the world around them. Both of these bred wild ambition in human beings, who spent their relatively short lives in both inquisitive and aggressive pursuits: learning about magic and building great empires.
This thirst for dominance caused them to be the center of power in Azeroth when the orcs were first summoned through the dark portal. This was was the beginning of the First War, and its consquences were so shocking for the humans that it came to be known as the pivotal moment that began the "Age of Chaos." The orcs rampaged through Stormwind and the regions around it, and within the space of 4 years, destroyed the city and forced the human refugees to flee across the sea to the north, to Lordaeron.
The end of the world, almost
During the Second War, the orcs pushed north to try and conquer Lordaeron as well, but the humans allied with the high elves and the dwarves to form the Alliance and drive the orcs back all the way to the Dark Portal, retaking Stormwind in the process. For almost 20 years it seemed that the worst disaster had been averted as humans stood triumphant with their new allies, and the remaining orcs were mostly relegated to concentration camps.
If your human character is now over 25 years old, he or she may have witnessed all these events as a child or young adult, and may carry an abiding fear of and hatred for the orcs who very nearly destroyed humanity. For a character around 20 years or less, however, this would have all taken place just before he or she was born and may have grown up with a more expansive sense of pride in her people, in the Alliance, and in their proven ability to dominate over the pitiful orcs and any other threat that stood in their way.
Any human character we might play, however, would have lived through the events beginning 5 years prior to the beginning of World of Warcraft, played out in Warcraft 3, and called "the Third War" according to the in-game lore. The undead came to power in the northern regions of Lordaeron and totally wiped out most of the humans and elves there. Those who survived or escaped that war were severely traumatized.
Undead and Demons and Orcs, oh my!
The undead summoned Archimonde and many demons of the Burning Legion at Dalaran, not far from Lordaeron city, but after that, most of the Third War took place across the sea to the west in Kalimdor. A human soldier or mage who took part in that war might have gone across the sea under the leadership of Jaina Proudmoore, met the night elves, and possibly even fought the Burning Legion on Mount Hyjal alongside the orcs, and witnessed the destruction of the night elves' world tree that ended the war. A human who stayed behind in the Eastern Kingdoms, however, would have only heard about the events in Kalimdor from other people -- and who knows how much of what they heard would be accurate, or how much they would choose to believe.
A human who fought alongside the orcs might have a grudging respect for them, while one who remained behind might still think of them as evil, simple-minded savages. A human who grew up in Lordaeron would have remained relatively safe during the first and second wars, only to lose everything to the undead 20 years later. A young human growing up in Stormwind after the first and second wars might have been raised in an atmosphere of pride, hope, and reconstruction, only to see the world outside of Stormwind seem to collapse around them during the Third War as he or she came into adulthood.
You may choose for your character to come from anywhere in the Eastern Kingdoms, in the areas around Lordaeron and Stormwind (such as Arathi or Westfall) but be careful not to stray too far into areas where there were no human settlements (such as Stranglethorn Vale), and do a bit of extra reading about your character's chosen hometown. Whatever you do, don't claim that your character was born and raised in Kalimdor -- humans didn't even know that Kalimdor existed until a few years ago, and modern human settlements there, such as Theramore, were only founded recently.
For further reading about the history of humans and the fate of their various kingdoms (of which Stormwind is the last remaining stronghold), have a look at the WoWWiki page on humans, as well as the Dramatis-Personae page on creating a human character. You might also find the Blizzplanet Warcraft Timeline useful as a reference.