Its mys older sister bday, shes turning 18. HELP to write her a letter?

I love her with all my heart she is like my role model and she lives in other country while she's in college, I really really miss her cause she's like my BFF we tell eachother everything and this is the first bday in all our lifes that we spend apart so its really sad :( but i want her to be happy in her birthday :) what could I write? some ideas please? im not very expressive or good writer :S