How to write an apology letter to a friend?

I'm trying to apologize to this girl because she means a lot to me. The last time we talked to each other was at high school and we haven't been in contact since. So now I'm trying to write to her an apology letter. I'm really unsure if this is a good letter to send because I never really had to write an apology letter before. So please help me by giving me some suggestions or tips on how to write a good apology letter.