There are actually many different ways to say "I" in Japanese (◕ω◕)♪
わたくし (Watakushi) - Very formal. Males or females.
わたし (Watashi) - Formal. Males or Females.
あたし (atashi) - Females only. Informal.
ぼく(boku) - Males only. Informal.
おれ (ore) - Males only. Very Informal.
Although you can often hear women in songs (and sometimes in anime) referring to themselves as boku, it is unusual to hear women say that in real life.