General

1. I want to know about Hinduism. What are the "ground rules" of this journey?

Whenever you are trying to understand any concept about Hinduism keep these simple facts in mind:

Hinduism is not a religion. It is among the oldest surviving societies that is amalgamation of centuries of wisdom, mythology, cultural influences and many cultures and languages within.
There are more than ten explanation and interpretation of each concept. Each interpretation will describe only a facet of the big picture.
There is a large loss of information, intent and conceptualization due to translation. That is why, if you find something that appeals to you, read other peoples translations too. If talking to a person, ask for examples and analogies. Each word or sentence is heavily overloaded. Make sure you understand the context very well.

Put it in another way, when ever you read something or hear something, remind your self:

OK. SO this is the view along this direction. Wonder how many more views are!
The original idea was put forth in a language (Sanskrit, Tamil, Hindi, or many of the other Indian languages) that differ from English drastically in how a sentence is formed, how sentences represent flow of thought and that use words that are overloaded with meanings.
I may not be getting the context or intent when I read the literal translation.

2. Now with these ground rules in mind, I am ready to listen. Tell me about Hinduism?

Word Hinduism is extremely overloaded. On context, it will refer to Hinduism as a

Philosophy (philosophy, science, mathematics), or a
Religion,
as a society (lately term Hindutva is being used to refer to societal aspects of Hinduism).

For a person born in India as a Hindu, is easy to extract the philosophy, religion and societal parts of events, practices and concepts. That is one aspect others often find is their biggest hurdle. How to see what concept is philosophy, what religion and what purely related to an ancient society.

On being bombarded with Hindu ideas, it may be worthwhile for the person to ask, am I to treat this information as a piece of philosophy, religion or reflections on a society.

Hinduism is a complex mix to these three or any permutation and combination of these. It is for a Hindu to decide how one wants ones Hinduism to be. That is the flexibility and respect for individuality this ancient society conveys.

3. Why is understanding Hinduism so complex?

Hinduism has evolved form/into a diverse society. To give you some perspective. Total number of languages up to 188 and dialects up to 544 dialects have been tabulated. Census respondents in 1961 provided names for 1,652 different "mother tongues." India was the favorite immigration destination over ages (2250 BC - Present). People from central Asia, middle east region of Asia, east Europe, east Africa, far east and Europe have come, settled and hence influenced the Indian culture. There is not one book or a life of one or few people to follow to get the whole picture. It is journey of life time.

4. If it is so complex, are Hindus so brilliant than even a child born in India understands Hindu concepts so easily?

:-). No, it is not like that. Being born in an environment makes it easy for someone to understand it's complexities and difficult to understand complexity of other other environment.