The first human Green Lantern was Hal Jordan, an airforce pilote who never played by the rules. After Abin Sur, the Green Lantern, who was defending sector 2814, crash lands on earth after being killed by an alien monster, the ring calls for Jordan, teleporting him to the California desert where Sur has crashed. Despite his disbelief that a human can make a good Green Lantern, Abin hands over the ring and dies.
Since Hal is a pilot, he adapts fairly quickly to flying. He's teleported to Oa after getting the ring for training at which point all the other aliens make fun of him because he's a human. His mentor becomes Sinestro (before he goes rogue) and Hal Jordan becomes the new Green Lantern of sector 2814.
Creation of the Green Lanterns
The Green Lanterns were initially created by the Guardians of the Universe. These Guardians are the oldest beings in the universe. They're little blue men with psychic abilities and are not only responsible for the Green Lanterns but also the evil that plagues it. Billions of years ago, the Guardians used to live on a planet called Maltus. They lived in a very intelligent society where they could develop their psychic and intellectual abilities. Everything was awesome until one of them, a Maltusian scientist who went by the name of Krona decided to go back in time to witness the creation of the universe.
Krona goes back in time and sees creation only to unleash evil across the galaxy. This results in his banishment and the Malturians decide to move to the planet Oa, a planet situated in the center of the universe. There they decided to call themselves Oans (Guardians of the Universe) and decided to create androids called the Manhunters. These Androids were powered by a central battery core in Oa. For milleniums, the Manhunters purged the galaxy of evil uintil one day a glitch in their system made them commit mutiny. They began slaughtering entire planets (which lead the survivors to hate the Guardians) before attacking Oa itself. After an epic battle, the Guardians took their power and banished them to the far reaches of the galaxy. This ultimately lead to a Manhunter planet where more Manhunters were constructed, a planet lead by Cyborg Superman.
The Guardians decided to choose new protectors of the universe, this time they would be human as to limit the risks of mutiny. They decided that they would choose the mightiest warriors from across the universe to aid them in their plight against evil. At first, these new guardians were equipped with just pistols that were powered by the central battery core. Later on though, they were equipped with rings and became the Green Lanterns. Like the pistols, each ring was powered by the central battery.
The first ever Green Lantern was Rori Dag and each Green Lantern is assigned with a sector he must protect.
The ring is a Green Lantern's best weapon. It can create anything the ring bearer chooses, all he has to do is think it. The ring is powered by the central battery core on Oa. For the user to make it work he must overcome his fear and turn it into positive energy (willpower). By doing this, the Lantern can use the ring. The ring can also make its user fly eventhough using the ring in intense ways can harm the person wearing it.
The ring recharges on the Lantern, a portable battery that the Lantern's carry around. The ring's only weakness is the color yellow. The ring can also serve as a translator (only at close range), an advisor and censors bad language.