key:
f/n = friends name
y/n = your name
Dear f/n,
I am writing you to inform you that I will be joining the ____ club at our school! I really like the whole concept of it, and maybe we could even join together? I just wanted to let you know in advance.
Kind regards,
y/n
(i don necessarily know why your sending this letter, so if you need anything changed, let me know!!)