Only email your teacher for school-related purposes.
Use an appropriate font: Ariel, Calibri, and Times New Roman are good.
Unless your problem is urgent, avoid emailing your teacher on the weekends or over the holidays. Teachers usually work full-time, and they don't get a lot of time off. They spend almost every day with their students, so respect the fact that they might need some time apart from their school and their pupils.
If it is a homework problem, write "ASAP" (as soon as possible) or "Urgent" in the subject bar. Be aware that some email spam filters will flag messages with all-caps subject lines.