Keep the email short and simple.Do not discuss your qualification in email.Ask for an appointment to explain better about your background.Do not send this email on fridays,saturdays or sundays.When professor returns on Monday his email is already swamped with other new emails.Neither Monday morning is a good time.Usually anytime weekday afternoon is good.
Dear Professor XYZ,
I saw the posting on student community center for employment as a graduate teaching assistant in your classroom for operations research. I would welcome being hired for this position. I am in a Masters/PhD program and expect to earn my degree in June XXXX.
I am writing this mail to apply for this position. I feel certain you have specific requirements and expectations for graduate teaching assistant at your department. I promise to meet them and to provide my best effort during my tenure if you decide I am the one to fill this job.
If we could meet in person, I could show you my work and find out more about what you are looking for. Please let me know what will be good time this week to discuss about my background. Thank you in advance for considering me for the position.