Hello, Hi is the short form of hello, we would use it casually with people we meet often. Hello is used when you want more emphasis on greeting with another person. Both forms can be used interchangeably and there is no strong expectation of hello over hi in formal situations. You will find yourself using both of them, sometimes you'll say hello, sometimes you'll feel more comfortable saying hi.

I agree with Wade. Sometimes it seems as though "hello" is a bit more formal than "hi". That being said, it is acceptable to use either one of them.

What is the difference between Hi and Hello? Hey is informal Hi is semi-formal Hello is formal