Watashi ( 私 ) and boku ( 僕 ) both mean "I ; me" - the difference lies in the usage by men and women. "Boku" is used by men. For example, if I said "Boku ha Porandojin desu", it would sound awkward to Japanese people because I'm a girl. :) If I'd like to say it right, I should use "watashi" (which is neutral) or "atashi" (used by women). Both "watashi" and "atashi" are written by the same kanji - 私.

Sensei is a tilte, attached to the end of a name: Suzuki-sensei; Honda-sensie; Yamasita-sensei. Kyoushi is "a teacher": Boku ha kyoushi desu.

kyoushi 一种の職業の呼び方. sensei 教師に対する敬称.