Hey Kayla, 

I know that to you, as an English speaker, this will seem very odd. I know that it did to me the first time I came across it. 

The comments above are good, in saying that "你吃了嗎？" is often used as an informal greeting between friends or people that know each other well and that are on a similar level socially, however is you've never met someone before it's not usually the first thing you'd say. I usually use (and hear) the following few greetings the most:

你好 - Hello/Hi/Hey

嘿 - Hey (direct transliteration)

哈羅 - Hello (direct transliteration)

嗨 - Hi (direct transliteration)

You will find that younger (and often people who are more familiar with the West) will simply say hi, hey or hello to you in English, especially in bigger and more modern cities. 

Alternatively, you could also just use someone's name, for example if my friend is called 小王, I would probably say something like:

Hey小王，你好！

The most commons things to ask after greeting someone are:

（你）最近過得如何？ or （你）最近過得怎麼樣？ How have you/things been recently?

最近還好嗎？Everything been alright recently? 

最近忙不忙呀？Have you been busy recently? 

你最近在幹嘛呢？What have you been doing recently? 

I hope that helps clear things up a bit.

Laurence 

哈囉我是雷雷

這句話其實是通用的問候，特別會用在用餐時間之後的見面喔~