Simply replace the unfilled areas such as "" and "[date]" with your information.
This letter is not easy for me to write, but your father that you know and love is not your biological father. When I was much younger I made some mistakes, one of which was becoming involved with [name of lover]. You were conceived as a result of this relationship and the greatest gift I could have ever gotten.
I am sorry that I have never mentioned this to you, but I have not felt that you were ready to hear this news until now. I also understand that you may have harsh feelings towards me for this life altering information, but know that I'm writing you this letter out of the love and respect that I have for you.
I have been in contact with your biological father over the years and if you wish to meet him it can be arranged, however that decision is entirely up to you. Please come to me for any questions you have for I am here for you.