It depends. If it's a picture postcard of your city, you can write about your place. You can talk about yourself, talk about something funny or just say hi! It's really up to you to decide, there are no constraints about that. Nonetheless, here are some tips for you to write about:

• your place
• yourself
• your personal homepage/weblog
• your country
• questions about the person/place/country you are writing to
• how to contact you back

Always remember to clearly write the provided Postcard ID. Only then will it be possible for the other person to register your postcard on the website once they receive it.