There are so many ways to say "I" in Japanese, I'll just have to link to another blogpost as they've already done the work. Knowing these may erase a bit of confusion if you're watching Anime.
Here are the ones I tend to notice most in anime:
私 Watashi is the standard for both genders, it's an abbreviation for Watakushi (now very formal).
僕 Boku has a soft connotation as it means man-servant (lol), there's a sense of humbling yourself before the speaker, it's more informal though.
俺 Ore is tough guy talk and very rude, though it's also used among good friends (no need to worry about being too polite).
Atashi is an informal female to say "I".
Next LJ post will be about "You"!