"Hello, I'm John" is very common in the US. Telling people your name is less important in the UK, where somebody else is likely to introduce you. Whichever way, you can not go wrong with this greeting with any English person. You can also use the word 'Hi', rather than hello.
More informal in the US is to say 'hi, how you doing?', which is often not seen as a real question. They do not really want to know how you are, beyond a simple response of 'good'. Then you can say, "Hello, I'm John" (if they do not know you) or just start a conversation.
Man walks into a bar...
Barman: 'Hi, how you doing?'
You: 'Good. Can I have a pint of beer?'
Man walks into a meeting...
You: 'Hello, I am John?'
CEO: 'Nice to meet you John, how long have you been in the US?'
You: 'We arrived yesterday, [insert polite observation about the place/culture/weather]'
More informal greeting should be kept to your friends, each clique will have its own informal greeting. Just stick to Hi or Hello until that is establish by the English speaker.