Americans use "How are you?" or "What's up?" as greeting phrases, depending on the situation. For example, if I was speaking to my boss, I would definitely say, "How are you?" With a close friend or even a stranger in a VERY informal setting, I would say, "What's up?", or even "S'up?", (leaving out the word what). These questions do not require any specialized answer - they are used a lot just when passing someone so that you have something to say. To be honest, most Americans aren't interested in your reply as long as it isn't too personal. If someone asks, "How are you?", you can respond, "Good.", "I'm fine, how are you?", or some similar phrase. If someone asks, "What's up?", it is appropriate to reply, "It's all good.", or "Nothing much." I hope this is helpful to you. I am developing a Youtube channel with videos showing unscripted English conversations. Check it out to hear more casual American English being spoken -https://www.youtube.com/channel/UCRA82OoEwRTjqxmxFeOAXiA