Your address, be that your business address or your personal address, should be written in the top right-hand corner of the letter. If using software like Word to write the letter, you might consider using a template to help you with spacing and indention. Do not indent paragraphs in the body of your letter. This is called a "block style".
The recipients address should be written on the left-hand side of the page, down and below your address.
The date should be included, especially on business letters. This can be put in two different places, depending on the writer's preference. You may put it on the right or the left-hand side just below the recipient's address. The month should be written out instead of using numerals. Double-space and then begin your salutation.
Your salutation or greeting should be formal, ie Sir or Madam, if you do not know your recipient's name. However, if there is a way to find out the name, do so. This puts a "face" to the letter instead of it reading generically.
Use "Dear (fill in the name) if you do know the recipients name. Use proper titles such as Mr., Mrs., Miss, Ms., General, ect.) followed only by their last name. Ms. can be used if you are writing to a woman that you are not certain of her marital status. Ms. can be used interchangably.
Make certain the body of your letter uses a block style meaning you should not indent paragraphs.
Your closing should be appropriate to the letter. Use "Yours Faithfully" to sign off if you do not know the name of the recipient. Use "Yours Sincerely" if you do know the name of the recipient. Add your own signature by typing it with enough space to add a handwritten signature between the close and your typed name. If you have a gender neutral name, it is wise to add your title in parenthesis just after your name, for example: Jaye Smith (Mr.)
Make a copy of your final letter, signature included, for your own files. It may be helpful later to remind you when you wrote the letter and what you wrote the letter about. This copy can also be helpful in a legal situation as proof of correspondence between parties.
It is also a good idea to consider the type of paper you write your letter on. Obviously, formal letters to businesses or organizations would need higher quality paper. The paper will make as much an impression as the words that are written there. Informal thank you letters or person-to-person letters can be written on regular printer paper or stationary.
Consider, as well, if you will want to include some sort of letterhead on your paper. If so, take a look at examples of letterhead or use a software template like the ones found on Microsoft Works. These can help you make your formal letter look more professional. Using letterhead for informal letters is not necessary.