Dear Harry,
I love you so much, and I'm so sorry I had to leave you. I didn't want to, but it was the only way to keep you and your father alive. If you're reading this, and your father did what I asked, you should be on your way to Hogwarts for the first time. I hope you are making lots of friends, and although no mother wants it, I'm sure there are a few enemies already. Don't worry, they aren't your enemies, they are the children of your father and my enemies. If that isn't the case, watch out.
I love you, Harry. I'm so sorry I couldn't watch you grow up. I have a feeling your father will have given you the cloak and map by now, and Padfoot's probably given you the talk. If your father hasn't told you stories about when he was in school, and got in trouble with me, ask him some questions about his fifth year with the Marauder's when you get home for Christmas.
Don't get into too much trouble, I believe there is a Weasley about your age, so try and make friends with him if you haven't already. His parents were real good friends with your father and I. I believe the son in your year goes by either Ron or Ronald. I also believe they had a daughter a few months ago. (From the time I am writing this, so she's a year younger than you.)
Don't get in Filch's way, say hello to Hagrid, don't break too many rules. Curfew is there for a reason, don't be out afterwards. Listen to the prefects and Head students. Stay safe. Don't go looking for trouble like your father. Moony and Padfoot ought to have seen you off with your father.
I love you so much, son. I wish I hadn't had to leave you.
Love,
Mum.
P.S. If you try out for Quidditch and make it, write your father and tell him your mum said "I told you so."