An informal letter is a letter you write to (future) friends, family or other people you know personally. They are more light-hearted than formal letters and luckily have less rules to them.
That doesn't mean there aren't any rules to follow.
First of all, keep in mind when writing an informal letter for school that it's still a school assignment so not everything you would normally write to your friends will be appropraie in class!
Here are some things to pay attention to:
Start the letter by addressing the person you're writing to. It's not a text message.
If you don't know the person, introduce yourself a bit.
If you received a letter from them first, be nice and thank them for it.
Try to use paragraphs instead of an entire letter written in one go. It looks neater and makes it easier to read.
Finish your letter with a closing sentence, for instance telling them you look forward to their reply.
End you letter with a last greeting such as 'best wishes' and don't forget to put your name underneath.
Short forms such as 'isn't' and 'I'm' are okay but try to stay away from slang such as 'gonna' and 'wanna'.