Japanese has (at least) two words for "teacher".
The word "sensei" is pretty well-known even among people who don't speak Japanese, but did you know that you shouldn't use sensei about yourself?
Here's what the textbook has to say:
"Use 'kyōshi' for yourself and the respectful 'sensei' for another person."
That's a pretty good starting point. But there's a bit more to it than that.
1. Kyōshi = school teacher
Kyōshi means the academic kind of teacher, someone who teaches in a school:
(watashi wa) kōkō no kyōshi desu.
I'm a high school teacher.
2. Sensei is a title
Sensei, however, is a respectful title, and should be used when talking about other people:
kare wa chūgakkō no sensei desu.
He's a junior high school teacher.
Watashi wa sensei desu is best avoided.
3. Sensei = master
Sensei can also be used more generally for a person who teaches something.
People who teach flower arranging or martial arts, for example, are sensei:
ohana no sensei
karate no sensei
sadō no sensei
teacher of tea ceremony
If you're talking about yourself, however, you still shouldn't go around calling yourself sensei.
You can use the verb 教える oshieru (to teach) instead:
(watashi wa) ohana wo oshiete imasu.
I teach flower arranging.
Certain types of professionals such as doctors or lawyers are also sensei (but again, not kyōshi).
Sensei is attached after teachers' names instead of san:
= Mr/Ms Yamamoto; “Yamamoto teacher”
It's pretty common to drop the name, too, and just call your teacher sensei:
Sensei, ohayō gozaimasu!
“Good morning, teacher”
So to summarise:
Use '“kyōshi” for yourself and the respectful “sensei” for another person.
“Sensei“ is not just for teachers, but also for masters of other skills, and for doctors
Affix “sensei” to your teacher’s name to show respect