This is your opportunity to tell the employer a bit about yourself, what level you are studying at and what experience you are looking for. Remember to be flexible and keen, at this stage getting a few days work shadowing may lead to further work experience offers.
Then you need to express your interest in the company and how this reflects in your career aspirations. Here you can highlight any key skills, interests and extra curricular activities that would relate to the company.
Remember to be professional and check for spelling and grammar mistakes. Leave a variety of ways for them to contact you such as a professional email and mobile number, state your availability and recognise that they will have a busy schedule also.
Sign off positively and thank them for looking at your application.