"How are you" is perfectly normal, standard English. Please don't think it's something serious and formal (or literary?). You can always use this without any problems. Of course, there are slangy ways of greeting and it's good to know these as well. However, if you always choose these phrases instead of regular English, you'll just sound like a poor speaker.

'How are you?' is not formal. It is a completely normal, neutral, everyday expression which you can use in any context. You can use it at any time , even with friends and family, and even in informal situations. More informal variants are 'How are you doing?' and 'How's it going?'. These are asking for 'progress reports' about someone's state of health, or their life in general. You might use these if you know that the other person hasn't been well or has had problems at work, for example. Other variants count as slang, or are specific to a region or an age group. In the UK, for example, an informal alternative is 'Alright?'. For example, if British two workmates pass each other in the corridor, one might say 'Alright?', to which the other responds 'Alright. You?'. No American would greet another in this way. And while it may be natural for some young people, particularly American youngsters, to say 'What's up?', it would sound ridiculous if an older person used this greeting. Those are two examples of variants which are only appropriate for certain people and certain places. Whereas you could be making a mistake using an inappropriate expression in the wrong context, 'How are you?' will always be fine. If in doubt, just use neutral English.

How are you isn't necessarily formal it is how most people who use forms of English close to standard greet people. In slang however "how you doing", "what's up", "how's it hanging" are common slang greetings but these are commonly used between close friends or people you like.

You could contract it as "How're you?" which sounds a little bit more natural, at least to me. As others have said, though, "How are you?" isn't formal. It can be used in formal and informal situations alike.

"What's new?" is another common informal greeting.