The attention line on an envelope indicates the intended recipient of a letter. The United States Postal Service shows that the attention line should always go at the top of the address instructions. It might say "John H. Doe," for instance.
When sending mail to a residence, the attention line is followed by the street address. Then the line with the city, state and zip code comes last. For a business letter, the attention line is followed by a line with the company name, and then the same address information on the next two lines. The actual word "attention" doesn't need to be included.