Let’s start with the first thing everyone see; the title of your thread!
You could make a title that says “penpal wanted” or simply “penpal”. But there’s a good chance that your thread will be viewed many times, while you get very few or no replies. And if people see a bunch of threads all called “penpal wanted”, “penpal” or similar, it becomes diffidult for the creators of all those “penpal wanted”/“penpal” threads to find a penpal. Simply because people get tired of looking through a bunch of threads without finding a “match”.
A better solution is to make your title unique and describe yourself and/or what you look for briefly in the title. “Mother of 3, looking for other parents as penpals” or “Animal lover, 18, looking for animal loving penpals near my age” or “Looking for penpals aged 18-25 from Asia”, “Female, 33, Norway, looking for Spanish speaking penpals from all over the world”. This way you can attract the right persons to your thread easily, don’t you agree?!
Then you’ll have to write the ad itself. People are already looking at your thread because they think you can be the penpal they are looking for. Use the thread to make them more interested, so that they actually contact you. Be honest and realistic, though. Don’t pretend to be someone you aren’t. Don’t give false impressions about how long letters you will write or how often you’ll write.
Here’s something you should cover:
Tell about yourself; how old are you, who are you, what’s your interests etc
Tell about what kind of persons you want as a penpal: age, gender, nationality, language, interests, etc… What matters to you? Some care about the age and gender, others don’t. Some might want to practice a foreign language or learn about life in another part of the world. Some might want to correspond with other parents or with other animal lovers or other health care workers…
Tell about your intentions and preferences related to the penpalship. Do you want to send letters or e-mails? How soon do you intend to reply; within 1-3 days after receiving the letter? Within 2 weeks? Or do you rather want to send one letter every week, even if you haven’t received a letter back? Do you prefer writing long or short letters? What’s your definition of a long/short letter?
Good luck with your hunt for a pen-pal!