First, start with "Dear ____"
Then, greet them. Say something like "Hey! Hope you are having a great week. I just wanted to tell you that my birthday is coming up, and that you are invited! Come over on _____ at ___ o'clock! There will be (birthday stuff; ie pizza) We'll do ____ and chill. It'll be awsome! See you there! From ____. Ps, if you can't make it I understand. Just call me or message me beforehand."
Have a great bday!
