the song is basically this line repeated 4 times: zhù nǐ shēng rì kuài lè (wish you a happy birthday) and shēng rì kuài lè by itself is happy birthday.
now how to read it....theres tones to consider but it goes something like this:
joo knee sheng ri kwai luh
the ri sounds like if you growled the r (you'll feel it kind of deep in your throat, theres a vibration)
better yet, theres youtube videos on pronouncing happy birthday you can check out
