Firstly, write a brief introduce yourself, who you are, what you are studying and what you are looking for in terms of a short 30- 70 hour placement, a sandwich placement, 10 week internship, etc.
Secondly, explain your interest in the company, the role itself and explain how this relates to your career aspirations. Give the employer some further reasons to take an interest in you (your interests and skills, other work experience or relevant training) remembering to relate this to the placement requested.
Finally, your closing paragraph should include your availability and that you have attached your CV. Your tone should be professional and ensure that you show your willingness to provide further information if needed and that you hope to hear from them soon.