From Nookipedia, the Animal Crossing wiki
To send a letter, the player must first obtain some stationery, which is sold in packs of four at Tom Nook's store (or Timmy and Tommy's store in Animal Crossing: New Leaf) for 160 bells. After the stationery is obtained and then selected in the player's inventory, the player must then choose the 'Write Letter' option. After the player has selected the recipient, the letter may now be composed. In Animal Crossing, Animal Crossing: City Folk and Animal Crossing: New Leaf, each letter has six lines to write on, whereas a letter in Animal Crossing: Wild World has only four. The lines containing the writer's name and the receiver's name can also be edited, but the recipient's name cannot be removed.
Once the letter has been written, the player is then given a complete view of the letter to allow for proofreading. Once accepted, the letter then appears in the mail section of the player's inventory. From there, it can be re-addressed, deleted, re-written or have a gift attached to it. All items except fish, bugs and turnips can be sent as a gift.
Once the player is satisfied with the letter, it must then be taken to the Post Office (Animal Crossing and Animal Crossing: New Leaf) or the Town Hall (Animal Crossing: Wild World and Animal Crossing: City Folk) to be sent for delivery. In Animal Crossing, when all five letters are sent in one go (the max that the post office can hold at one time), Pete the mailman can be seen flying into the air when the player leaves the building. After he has flown off, the post office is empty and the player can again send a maximum of five letters.
Available recipients include the following:
If a letter containing significant spelling and grammar errors is sent to a villager, this may result in a complaint by the villager.
Letters have a present attached only if one is sent to the recipient.
Depending on how many lines players write a neighbor, they can give them different items in return. For example, if players give them a piece of fruit and write them a one line long letter, they will give the player a piece of fruit back. This can be any type of fruit.