The Manager [name of the addressee]
[Details of the Designation]
[Mention the address of the company here]
[Mention the Date:]
[Subject of the letter should be mentioned]
Dear Mr. Name or the Surname,
The first, initial paragraph is the introductory paragraph. It talks about the purpose behind the letter, like for instance application of job or enquiry etc. If you are enclosing your resume, make a mention about it in the first few lines. If you have met the person in charge then make a mention of it, or if you have any recommendations from any end, specify the name of the person or company.
The second paragraph usually talks about your professional background, like the professional and educational qualification one has. Pour some light on your interest levels to take up this job and why you wish to be associated with the particular company. If you know that you are the perfect choice for the job, state the reasons why you feel so.
The last paragraph is the sign off one. This is where you mention about hearing from the addressee soon.