Hi. Some suggestions below:
A letter to my friend [ / A letter to a friend / A letter to a friend of mine (assuming you have more than one friend) ]
Dear Lily,
Next Saturday is my birthday. I am going to have a party and celebrate my birthday at eight o'clock pm. I want you to come [ / I want you to be there too]. First you'll need take the No.--- bus and get off at Bank of Jiaxing and the then you can go on Xingqiao Road. You will see a red house on the left side of the road after five minutes. I will wait [ / be waiting ] for you.
Yours,
Li Hua