[I'm not a teacher.]
Welcome to the forums.
1. State your purpose of writing the letter - Why do you write?
2. Offer your bank account(s) info. so that bank staff can follow and check.
3. Attach your contacts.
Here's the format of letter. You try to write it, and then post it up for people's comments.
Formal Letter Writing - Articles - UsingEnglish.com
Hope this helps.