After you roll your ability scores and before you write them on your character sheet, choose your character's race. At the same time you should also choose a class, since race affects how well a character can do in each class. Once you know your character's race and class, assign your ability score rolls to particular abilities, alter those abilities according to race, and continue detailing your character.
You can play a character of any race and class combination, but certain races do better pursuing certain careers. Halflings, for examples, can be fighters, but their small size and special features make them better as rogues.
Your character's race gives you plenty of cues as to what sort of person he or she is, how he or shee feels about characters of other races, and what his or her motivations might be. Remember, however, that these descriptions apply only to the majority of each race's members. In each race, some individuals diverge from the norm, and your character could be one of these. Dont let a description of a race keep you from detailing your character as you like.