There is no one set format for an informal letter, but there are some common elements that you will find in most letters. For example, most letters will begin with some sort of greeting, such as "Dear so-and-so." The body of the letter will usually be relatively informal, and will often include updates on what's been going on in your life, as well as news about mutual friends or family. The letter will usually end with some sort of valediction, such as "Love," "All the best," or "Take care."