As far as American application letter is concerned it is like simple application letter that your write whenever you are seeking job, admission, service etc.
However, you can follow the following format as far as American application letter is concerned.
Name of the Sender
Address of the Sender
Name of Recipient,
Name of Department,
Name of Institution,
Sub- Write the relevant subject of the application
First paragraph- in the first paragraph you need to apply for the relevant thing. For e.g. Job, Admission, etc.
Second Paragraph- who are you, what you have done and why you are making application
Third Paragraph- Ask for the due consideration of application and request for the prompt reply
Name of Sender