Letter which is written to thank your friend for his friendship is called Friendship letter. Given below is a sample which will make you understand how a friendship letter should be written.

To:
[your friend's name]
[Street address]
[city, state, zip/postal code]
Date MM/DD/YYY.

Dear [Your friend's name]

Its almost impossible these days , to find an amazing friend like you. There is nothing which i have kept away from telling you as i completely trust you. I’m also happy that you trust me the same way i trust you. I have never had happiness and joy elsewhere like i have with you.
So today, i want to thank you for being a true friend to me who i can rely on.
Thank you so much.

Your best friend
[your name]