When I was a freshman, one of the scariest and most uncomfortable things was trying to email my professors. The main trouble was to keep formal style without turning to informal letters like those I write to my close friends:
Howdy, Jack! Sorry but I can’t visit your class tomorrow, that’s why it would be nice if you could send me the material I’ll miss.
It wasn’t that horrible, but all in all I had to learn some formal writing rules to cope with the problem of easy communication with professors.
Syllabus for the beginning
To start with, you have to find your professor’s email. Usually it is placed on the top of your syllabus front page. Another way to find address is to contact anyone who attends the same class or to visit your college website.
For such mailing I offer you to create kind of a formal mailbox(if you still don’t have it). Be sure that letter received from firstname.lastname@example.org would look better than the one from email@example.com.
In most cases professors tell their preferences on how they want to be called (usually – first class). If that didn’t happen, try using formal writing. When you surely know their title, call them like Dr. or etc. If not, the word professor before the name would be rather formal and suitable.
Try to start letter with wishing good morning or good day, try to apologize for distracting, but don’t write too much in your greeting. Be pleasant and nice, especially is you are asking for a favor or advice later. A good practice is to add your class (let’s say you are Todd Jenkins from Literature 503).
Good email is brief. You can easily give full topic coverage and give all the necessary details, but in most cases person is not really eager to read huge letters with much explanation. So try to be brief, tell basic idea and make your writing readable. Anyway, you can be asked for any additional information later (if there will be something that needs more detailed coverage).
In the end of the letter thank for help, wish a good day, show gratefulness for the time spent and attention. The best way to communicate with your professor is to show them that the subject is really important, you appreciate their time and the help you get during the classes.