there are some rules in order to write formal letter
1- Use full not contracted forms like I am not I'm
2-Don't use simple words like find out but use discover......and so on
3-End with your faithfully when you don't know the name of the receiver
4-End with your sincerely when you know the name of the receiver
5-Put the name and the address of the receiver on the left
6-Put the address and the date of the sender on the right