Most people looking for penpals like to have a pretty good idea of who they'll send the message to. To which extent the likes and dislikes match up and in general the writing style of the person. Here's an attempt to help people who are not very good at writing free form about themselves.
As with everything, all that I say here is highly subjective, while "most people" like to have a good idea about the person, there are many who also enjoy the process of getting to know someone from the groundup and prefer that over a more general introduction. These are just guidelines, follow them at will!
Remember this is just a suggestion, feel free to submit your post as you prefer!
Title format: [Age/Sex/Location] What you are looking for
About: General information about you, the first sentences or words that would come to mind if someone asks "who are you?"
Hobbies and interests: Self explanatory, how you spend your time.
Favorite books/movies/shows/etc: Also self explanatory.
Requests: In case you're looking for a specific type of person.
Happy writing! Remember to tag your post with the proper flair!