The first word every learner needs is 你好 (nǐ hǎo) - Hello. It's used in almost any situation. To be more polite, you can add a title. For example, to a teacher, say 老师好 (lǎoshī hǎo). To say goodbye, you can use 再见 (zàijiàn). A more casual way to say "bye" is 拜拜 (bāi bāi), which sounds like "bye-bye"!