In a polite formal way, it would be:
Happy Birthday = お誕生日おめでとうございます。
I like you very much = あなたのことがとても好きです。
but since it's your best friend, it would be better to write it more frankly as:
btw, "Happy Birthday" can also be written as "ハッピー・バースデイー" (the same pronunciation but in Japanese) because it's a common phrase.