“happy birthday” in Japanese is “tanjoubi omedetou” or "tanjoubi omedetou gozaimasu,” but which of the two sayings you should use depends mostly on to whom you are speaking.
"tanjoubi omedetou" is the more casual way of saying Happy Birthday
You should only use this to people you are familiar with like family, friends and classmates. YOU SHOULD NOT use it to anyone with a higher status than you.
The kanji for this phrase is:
This is the more formal polite way of saying Happy Birthday.
Gozaimasu means "very much"
The full kanji for this expression is 誕生日おめでとうございます