Write a little background about yourself - your interests and hobbies, what you like to do in your spare time, your studies, work, pets, your favorite movies, what bands you like, what books, what religion or philosophies interest you, etc.   Avoid sharing too much personal information.

Be upfront about your ability to write on a regular basis. If you are only able to write once a month, let that person know so that he or she doesn't look for your letter and feel that you are not interested.

Greeting cards can be a good way to make initial contact. There are so many friendship-type cards available just to say "hello" to the prisoner.  This can take the pressure off of you worrying about what to write that first time.

Prisoners are happy to get your letter(s) and are looking for words of encouragement.

Make sure you put your return address on the envelope (be sure it is legible) and include it again in the body of the letter in case something happens to the envelope.  Most prisons won't accept letters without return addresses.  If you don't want the prisoner to know your home address, get a P.O. Box.

You might want to include a photograph of yourself so the prisoner has a "face" to put with the name.  Many prisoners are forthright in stating they are looking for relationships, but others are simply looking for a friend with whom they can correspond.  A photo would be a nice gesture of friendship.