The basic rules and regulations for the NBA draft are that the player has to be a minimum of 19 years old, and players who are not international players should be at least one year past their high school class graduation.
Players who are not automatically eligible for the NBA draft must declare their eligibility to be drafted by letting the NBA headquarters know about it, in writing, at least two months before the draft. Players are automatically eligible if they are at least 22 years old and have completed college or university (for U.S. players).
International players must have never attended a United States college or university, have been a permanent resident of a nation that isn't the United States for at least three years before the draft (while playing basketball), and have not attended or completed high school in the United States.
Players who are at least 19 years old (or one year removed from high school graduation) and who do not qualify as international players are automatically eligible for the draft if they have finished their college eligibility, if they completed high school in the United States but did not enroll in college and if they didn't finish high school in the United States or if four years have gone by since their high school class graduated.
NBA draft rules and regulations for eligibility change almost every single year. If you are a basketball player who is interested in the NBA draft, you should keep yourself apprised of the newest rules and regulations.