Another name for informal letters is 'Friendly Letters'.
When you are told in the exam to write a letter to say your father, mother, sibling, uncle, aunt etc. or a friend, the type of letter structure expected is that of the Informal Letter.
These types of letters usually have six parts:
1. The Heading
This is made up of the writer's address and the date.
2. The Greeting
The greeting always ends with a comma:
Dear Kwame,
Hello Mary,
3. The Introduction
I'm so happy to hear...
How is the business...
It's been a while since...
4. The Body
This is the part of the letter that should contain the reasons why you are writing.
Use literary devices/figurative expressions to make your letter more colourful.
5. The Complimentary Close
My regards to your family
I've got to run now
Looking forward to your reply
6. The Sign-off
Love,
Joe
Regards,
Abena
Best wishes,
David