Dear (Mrs./Mr./Miss/Ms.) followed by last name,
In your introductory paragraph, clearly state your reason for writing. If you are writing to inquire about an open position, explain that your qualifications make you an ideal candidate for the job. If you are writing regarding products, services or for some other reason, make it clear from the beginning.
The body of your letter should describe the details about why you are writing. Explain your interest in a product or service, how it would benefit you or your organization; or, let an employer know a bit more about your skills and achievements. When writing in regards to a job opening, include only information targeted to the specific job.
In closing, give the recipient any information that would further allow them to contact you. Explain that you look forward to hearing from them. If you are writing about a job opening, explain that your resume is enclosed, and that you would like to discuss your qualifications and how you could benefit the company.
Thank the recipient for their time and consideration.
As you can see, writing an inquiry letter is fairly simple and straightforward. Keep your letter brief and on topic, avoiding lengthy details.