It's true that Hiya is also used a lot. Just respond with "Hi! or "Hey! or something similar informal.

There is no reason to feel uneasy if someone says How do you do.
Just shake their hand and repeat the phrase back.
That's all you need to do.

The Hindu : How do you respond to "How do you do"?
It depends on which side of the Atlantic you are from, I guess. People from Britain usually respond by saying "How do you do?". For them, "How do you do?" is an expression which is normally used when you are introduced to someone for the first time. "How do you do?" is not the same as "How are you?" For the English, the response to "How do you do?" is "How do you do?" It's like our "namaste". When somebody says "namaste", we respond by saying "namaste". Americans, on the other hand, think differently. You will find that many Americans do not always make a distinction between "How do you do?" and "How are you?" So don't be surprised if an American responds to a "How do you do?" with a "Fine, thank you", or "Doing good".
An English person would get a very high opinion of a foreigner who responds correctly to this questions. In the USA I really wouldn't know what to do. The reason the Americans don't respond correctly is probably because they are not familiar with this British expression. In the UK it's considered rather unsophisticated to say anything other than the same phrase back. (I know, because I've done it and been told off by my then fiancee). Up to you what you prefer!