James was probably born at around mid 1960. He is a pureblood, born to rich elderly parents, who had him very late in life and saw him as their special treasure. James is therefore quite pampered and spoiled, used to getting what he wants.
He went to Hogwarts, where he supposedly met his three best friends: Sirius, Peter and Remus. James and Sirius became almost brothers, counting on each other for anything until then end of James' life. The four boys became close friends and took to using each other's intelligence (especially that of James and Sirius, who were exceptionally bright), to play pranks and cause trouble, they dubbed themselves the 'Marauders.'
In second year, James, Peter and Sirius discovered that Remus was a werewolf. Wanting to help their friend, the three boys started working hard to become Animagi. They succeeded in their third year. James could now become a stag at will - An extraordinary achievement at the age of 15. Now the boys took to exploring the school and Hogsmeade during full moon nights. They became very much acquainted with these places, and that is how they came to write the Marauders Map.
James is also a gifted and natural flyer, he is Gryffindor's star Chaser and worshiped by many at his school for that reason. James and Sirius are also admired at school for their talent with magic and their intelligence. This often makes them get a little too carried away for their own good.
James started noticing Lily Evans in fourth year, and being the not-at-all-shy person that he is, started asking her out at every available opportunity. Lily refused, however, claiming that James was too much of an arrogant, bullying toerag for her to accept. Being the persistent wizard that he is, James never gives up. Good thing too, because Lily doesn't really hate him.
James and Sirius share a mutual loathing for the loner Slytherin Severus Snape who, much to James' disgust, is immersed in the dark arts. The two boys have expressed their loathing by publicly humiliating Snape. The hatred for Snape, however strong, did not stop James from saving the slytherin from Remus' werewolf form in sixth year, when Sirius recklessly sent Severus into the whomping willow.