Hi Parham... first it depends how "formal" you want to be, and also some greetings may depend on whether you're english, american or australian.... All the greetings mentioned by Curie and TJ are correct but they are very informal - something you can use with friends but that you probably would not want to use in business meetings, with your boss, teacher or mother in law! In these more "formal" situation stick with the "Hello" "How are you?", "How have you been?".....

You can just say "Hey, how are you?". "Hey" is informal enough, it will be understood by people everywhere and it won't seem too out of place if you use it in the wrong context. With greetings, context is everything. Greetings can differ greatly by region, age group and formality. For example, "How are you going?" is a great all-purpose greeting in Australia (people often say it like "Howuhyu going" informally). It's very common here, but if you said it to someone from the US or UK you might get a blank stare. I didn't even realise it wasn't used in the US until an American asked me if I meant "Where are you going?". Someone told me it's used in parts of Ireland, but I can't confirm that. "G'day" is another Australian greeting. As far as I know it's only used in Australia (possibly New Zealand and South Africa as well), but it's well recognised overseas as an Australian greeting. The thing is, it's actually very rarely used in cities in Australia. It's much more common in rural areas, but in urban areas you'll find it's mostly used by tradies. My dad didn't start saying "G'day" until he lived in the US because he was sick of people thinking he was British! "What's up?" is a common informal greeting in the US, but here it's something in between "What's wrong?" and "What's the news?". If I call someone on the phone, they might ask "What's up?" after we've both said how we are. In this case, "What's up?" really means "Why are you calling me" (although it sounds much friendlier). This usage is informal. People around my age commonly ask "How's it going?". This is a habit picked up from American movies and TV. "How's it going?" rolls off the tongue quicker than "How are you going?". This is pretty informal though, and it would sound strange to me hearing someone older or younger than me say it. There's nothing unnatural about "Hey, how are you?". As an all-purpose informal greeting it works just fine

In my personal experience, I've used and heard the exchange of "Hello, how are you?" "I am fine, thank you. How are you?" as the following: "Hey, what's up?" "Not much. You?" "Same." Other greetings: "Sup?" {This is essentially just "What's up?" without the "What" part.} -a smile and a quick wave of the hand- {This nonverbal greeting is usually used if two people are just passing each other by without the intent of stopping to have a conversation.} "Morning"/"Afternoon"/"Evening" {The word "Good" is sometimes added before each greeting: "Good morning," for example. These are somewhat in between informal and formal, I would say. These greetings would be appropriate for most everyone. The "Good" added before makes the greeting a little more formal, though.} All of the greetings given by Curie in her post are also used.

Ways I would greet people when speaking would be Hey Hi Hiya How you been? A greeting that is very British is "(You) alright?"