Informal:
* Привет! (Privet!) - This is the most common and versatile greeting, similar to "Hi!" or "Hello!". It can be used with friends, family, and acquaintances.
* Как дела? (Kak dela?) - This literally translates to "How are things?" and is a common way to ask "What's up?"
* Как жизнь? (Kak zhizn?) - This means "How's life?" and is slightly more informal than "Как дела?".
* Что нового? (Chto novogo?) - This means "What's new?" and is a good way to ask about recent happenings.
Formal:
* Здравствуйте! (Zdravstvuyte!) - This is the most formal greeting, similar to "Good morning" or "Good afternoon". You would use this when addressing someone you don't know well or someone older than you.
* Как ваши дела? (Kak vashi dela?) - This is the formal way to ask "How are you doing?" and is used with strangers or people you respect.
Other options:
* Всё в порядке? (Vsyo v poryadke?) - This literally means "Everything is in order?" and is a good way to ask "Is everything okay?"
* Как настроение? (Kak nastroenie?) - This means "How's your mood?" and is a more casual way to ask "What's up?".
The best way to say "What's up" in Russian depends on the situation and your relationship with the person you are speaking to.