Refer to your professor by the title "Professor" or "Dr.". You should never address them by "Mr." or "Ms.", because some people consider it insulting due to the fact it does not acknowledge their credentials. Likewise, you should avoid the term "sir", since in the U.S. it is considered impersonal and also ignores the fact the person you are addressing has an advanced degree. (The term "sir" is sometimes used in other countries to address teachers or professors, but it is not considered common or proper in most U.S. universities or colleges.)
If your professor has a Ph.D, you should address them as "Professor LastName" or "Dr. LastName". If they do not have a Ph.D., or if you are not sure, address them simply as "Professor LastName".
