"How are you doing?" (Formal) "What's up?" (American informal) "Alright." (British informal) "Yeah." (Informal, used often) "Nah." (Informal, not used often but not rare either)

Other options Americans use : How's it going? What's going on? How are you? We use these to say hi. No one ever expects an answer. So if you are just saying hi, you can use these or also hello, hi, hey Yep Nope (these can be used when you want to be silly with your friends. Only use them occasionally)

... just a small but important correction to @DR HAlll's suggestion about the informal British English greeting. Like 'How are you?', the greeting 'Alright?' is a question, said with rising intonation. 'Alright' in the non-question form is the response. The exchange might go like this: 'Alright, mate?' 'Alright. You? 'Yeah. Alright.'

That's all. And thank you