Here are some letter ideas to get you started:
- pretend you are writing to the love of your life and write all the things you would like to say to them.
- make some images if you don't know what to say.
- concoct a grand plan which you know you will never follow through with.
- give details of really awkward or embarrassing experiences
- describe a recent dream
I hope that is enough to inspire you and get you started.