How to say "what's up?"

你可以说：嗨，你最近在忙什么？

When you want to sound more informal with your friends in Chinese, you can say "Zěnmehuìshì?" (怎么了？) or "Jìnkuàngrúhé?" (近况如何？). These are more colloquial and casual ways to greet or ask about someone's well-being.
So, if your friends say "what's up?" to you, you can respond with "Méishénme, nǐ ne?" (没什么，你呢？) or "Háihǎo, zuìjìn zěnmeyàng?" (还好，最近怎么样？). Using this kind of language will make your conversation sound more natural and relaxed.
Tone and facial expressions are also crucial in casual conversations. Speaking in a light, conversational tone and smiling naturally will enhance the friendliness and enjoyment of your communication.
Hope this helps you connect more casually with your friends!

怎么了可以换成 “咋了”？当然主要不在于用什么词，而是表情：你说怎么了或者咋了时要表现出焦急，关心的态度，而不是冷漠。朋友说你正式是想说你表情有点冷漠。

Hey, you can use this phrase: 你在干嘛？or 干嘛？it similiar with "what's up?" in Chinese.

吃了吗？
Have you eaten?
好久不见
Long time no see

you can say "最近怎么样？" or "一切都好吗？" or "最近忙吗？都好吗？" to instead of "你好"

“怎么”连读说的快一点，”了“发轻音

咋啦 ？ 咋的啦？ 最近咋样？ They are less formal but more like the northern dialect in China.

怎么了?
什么事?
有事吗?

To achieve a more colloquial and authentic speaking style, in addition to word choice, it is important to consider factors such as intonation, tone, and body language. Consider the difference in tone, demeanor, and facial expressions when greeting someone with a formal handshake and saying "hello" versus casually patting a friend on the shoulder and saying "What's up?"

Therefore, to make your speech more colloquial, my suggestion is to first relax yourself when greeting others, and then incorporate appropriate interjections and fillers. For example, saying "你好" (nǐ hǎo) in a serious and rigid manner compared to saying "Hi, 你好" (Hi, nǐ hǎo) with a relaxed and upwardly curved mouth, the latter will create a more informal atmosphere. Adding a modal particle suffix “啊”（a） to express "what's up" can be said as "你（最近）怎么样啊～？"（nǐ zuìjìn zěnme yàng‘a？）“怎么了啊？”（zěnme lea？） which does not sound formal at all.

Hope this helps.