What is   Sanatana Dharma?

Sanatana Dharma also known as Hinduism means eternal and timeless law, that is way of life.

Eternal means that has, is and always will be.

Dharma and religion is not the same.

Religion is beliefs. Beliefs of the things that you do not know about.
Example: Reading and talking about water.

Sanatana Dharma is about seeking and experiencing. You don't have to believe it, if you don't want to. You can and have to experience it yourself.
Example: To experience water, you have to find and drink the water.

Sanatana Dharma is a tree springing from the core concept of dharma and splitting up into innumerable gurus (teachers)/schools/practices as of leafs of tree.

Sanatana Dharma gives freedom of thought, it's a vibrant Philosophy with many noble ideas and intelligent thoughts.

Where to start?

There is no 'ism' or ideology in Sanatana Dharma (what you call as Hinduism). Several ideas exist in this dharma. All of them tell you to live life in a detached way, becoming a witness of yourself, analyzing your thoughts, rightful Karma (actions & consequences in other words cause and effect of your actions as well as inactions) thereby deriving multiple perspectives and solving problems of life.

To start for beginners, read the Bhagavad Gita (important to choose original and authentic version, some good version are from Gitapress or Ramakrishna Mission or Srila Prabhupada...), which is one of the most important texts in Hinduism. It's a dialogue between Lord Krishna and Arjuna that provides insights into the nature of reality, the purpose of life, and the importance of dharma or righteous action.

Another great read is the Ramayana , which is an epic tale that tells the story of Lord Rama and his quest to rescue his wife Sita from the king Ravana. The Ramayana offers insights into the nature of dharma, devotion, and the importance of living a virtuous life.

Additional great read is the Upanishads , which are a collection of philosophical texts that explore the nature of the self, the universe, and the divine. The Upanishads offer profound insights into the nature of reality and the path to self-realization.

Now, if you're interested in delving deeper and deeper into Sanatana philosophy, read the Ashtavakra Gita, which is a dialogue between King Janaka and the sage Ashtavakra. It explores the nature of the self and the path to liberation, and offers profound insights into the nature of reality and consciousness. This is not for beginners though.