The first ever Chinese song you should know. The easiest Chinese song ever cuz it has only one sentence. Probably the most useful Chinese song too!

shēng rì kuài lè gē
生日快乐歌

Happy birthday to you 祝你生日快乐 zhú nǐ shēng rì kuài lè
Happy birthday to you 祝你生日快乐 zhú nǐ shēng rì kuài lè
Happy birthday to you 祝你生日快乐 zhú nǐ shēng rì kuài lè
Happy birthday to you 祝你生日快乐 zhú nǐ shēng rì kuài lè

祝zhù, wish 你 nǐyou 生日shēng rì快乐kuài lè happy