The best-known word of greeting is 你好(nǐ hǎo), a both-third tone in which case the first thrid tone should be pronounced as a second one. If you know nothing about pinyin system, it may sounds like "knee how".
But you may find it actually not so often used in real life. A real Chinese would speak as below (in different occasions):
嗨!(hài) - Just the transliteration of "Hi!"
早上好! (zǎo shàng hǎo) - Good morning!
大家好!(dà jiā hǎo) - Hello Everyone!
最近怎么样?(zuì jìn zěn me yàng)- How are you these days? -- A greeting to an old friend.
去哪儿?(qù nǎ er) - Where are you going? -- A Chinese greeting that’s commonly used when you run into someone.
and so on.