A business letter is a formal letter with six parts:

The Heading
The heading contains the return address with the date on the last line. Sometimes it is necessary to include a line before the date with a phone number, fax number, or e-mail address. Often there is a line skipped between the address and the date. It is not necessary to type a return address if you are using stationery with the return address already imprinted, but you should always use a date.  Make sure the heading is on the left margin.

Example:
Ms. Jane Doe
543 Washington St
Marquette, MI 49855
Tel:
Fax:
Email:
June 28, 2011

Recipient’s Address
This is the address you are sending your letter to. Be sure to make it as complete as possible so it gets to its destination. Always include title names (such as Dr.) if you know them. This is, like the other address, on the left margin. If a standard 8 ½” x 11” paper is folded in thirds to fit in a standard 9” business envelope, the inside address should appear through the window in the envelope (if there is one). Be sure to skip a line after the heading and before the recipient’s address, then skip another line after the inside address before the greeting. For an example, see the end of this sheet for a sample letter.

The Salutation
The salutation (or greeting) in a business letter is always formal. It often begins with “Dear {Person’s name}.” Once again, be sure to include the person’s title if you know it (such as Ms., Mrs., Mr., or Dr).  If you’re unsure about the person’s title then just use their first name. For example, you would use only the person’s first name if the person you are writing to is “Jordan” and you’re not sure if he or she is male or female.
The salutation always ends with a colon.

The Body
The body is the meat of your letter. For block and modified block letter formats, single space and left justify each paragraph. Be sure to leave a blank line between each paragraph, however, no matter the format. Be sure to also skip a line between the salutation and the body, as well as the body and the close.
               
The Complimentary Close
The complimentary close is a short and polite remark that ends your letter. The close begins at the same justification as your date and one line after the last body paragraph. Capitalize the first word of your closing (Thank you) and leave four lines for a signature between the close and the sender’s name. A comma should follow the closing.

The Signature Line
Skip at least four lines after the close for your signature, and then type out the name to be signed. This often includes a middle initial, although it is not required. Women may put their title before had to show how they wish to be addressed (Ms., Mrs., Miss).
The signature should be in blue or black ink.

Enclosures
If you have any enclosed documents, such as a resume, you can indicate this by typing “Enclosures” one line below the listing. You also may include the name of each document.

Format and Font
Many organizations have their own style for writing a business letter, but here  are some common examples.

Block
The most common layout for a business letter is called a block format. In this format, the entire letter is justified to the left and single spaced except for a double space between paragraphs.

Modified Block
Modified block is another popular type of business letter. The body of the letter and the sender’s and recipient’s addresses are left justified and single spaced. However, in this format, the date and closing are tabbed to the center point.

Semi-Block
The least used style is called a semi-block. In it each paragraph is indented instead of left justified.

Font
The standard font for business letters is Times New Roman, size 12. However, fonts that are clear to read such as Arial may be used.

Sample Letter

{NOTE: your name goes only at the bottom}
Your Return Address (no abbreviations for Street, Avenue, etc.)
Your City, YO [your two letter state abbreviation] zip
Date (write out either like June 4, 2004 or 4 June 2004)
First and Last Name of the Person to whom you are writing
Address
City, ST zip
Dear Mr./Ms. Whomever:
In the first paragraph, introduce what you are writing about and what you want from them.
In the subsequent paragraphs, explain the nature of your problem and what they can do for you. Be non-combative and straight to the point.
In the last paragraph, be sure to thank him/her for his/her time and efforts on your behalf. Also, let them know that you will contact them or that they can contact you with any questions.
Sincerely yours,
{four spaces so that your signature may appear here}
Jane Doe
A business letter is not restricted to one page; the letter should be as long as it needs to be.

source: https://www.nmu.edu/writingcenter/parts-business-letter [Northern Michigan University]