Answer:
firstly u should know the format of informal letter which is : i have uploaded a pic so u can see format from there now lets start with the letter.
Explanation: Dear ......,
Hi! How are you? I am as fit as fiddle. The reason I am writing this letter is to inform you about my studies and how are they going. We started school together so I felt like sharing my progress with you. surprisingly, I am doing really well in my studies as you know I was not very good in studies in the previous grade. But later on after getting my result I was very ashamed and I started hardworking and new teachers are also really really good. when there is a complain in understanding anything the friends also are very helping you shifted from here to abroad but still it doesn't matter you were a good friend and still is so I love to share my progress with you. Nowadays are going on really good I wish we both were together but no worries .I would be really happy if you share about yourself and how your studies are going on.
Awaiting reply.