what's up?= what's going on?天天见的人没什么新鲜的啊,就可以说not much或者nothing much；或者抱怨一下my girlfriend is pregnant...或者说点高兴事my girlfriend is pregnant!或者有的时候就是单纯的用来打招呼,你也可以用"what's up?"回复他；具体情况具体分析.
How are you?= How are you doing?是问你进来如何啊?回答的时候一般其实省略了前面的"I'm doing ..."所以可以说（包括但不限于）fine/good/well/ok/pretty good/just fine/great/not good/not bad/awful.这些用法我都听人说过.之后可以thanks可以问how are you?或者you?...或者直接跟人开聊近况.看心情.