What do I call the teacher?

Instructors are addressed as “Sensei”. Sensei is a Japanese title of respect (sort of like “Sir” or “Mister”). It means “teacher” and is literally translated as “one who has gone before”. This is in reference to that persons training and overall experience.
“Senpai” (“senior”) is another term of respect that is often used when addressing someone who is not an instructor, but is senior to you in karate experience. “Kohai” (junior) is used when addressing someone with less experience.