There are several forms of the word 'I' in Japanese.

Watakushi: The most formal way of saying I.
Watashi: Less formal than above.
Boku: Used in informal conversation. Usually used by males only, although some hip girls are using it nowadays.
Ore: Used in even more informal context. Again, males only.

Watashi is the word for 'I' first taught to people learning Japanese.

The difference in male and female speech is only in informal speech. It's like in English: A female using male speech is considered, uh, 'butch', and a male using female speech is considered 'a fruit'.