For anyone new, Bhagavad Gita, writings of various scholars and saints are great ways to start. My favorite is the writings of Swami Vivekananda (and Ramakrishna Mission in general). Additionally, there are tons of videos on youtube from Swami Tejomayananda of Chinmaya Mission, they are great too. 

For the people from Maharashtra, Dnyaneshwari is the best Bhagavad Gita anyone can ever get. So, don’t miss that. 

I wouldn’t recommend reading Upanishads to start with for anyone (they are great once you cross basic understanding). 

Having said that, don’t go with what I say alone, explore your path. Once you start, you will know what’s best for you. 

However, is reading alone enough? As we saw during our education, we mostly forgot what we read once the exams were over, right… So, there must be more steps needed, beyond just reading. 

Brihadaranyak Upanishad provides the best advice on that.
आत्मा वा अरे द्रष्टव्यः श्रोतव्यो मन्तव्यो निदिध्यासितव्यो मैत्रेयि, आत्मनो व अरे दर्शनेन श्रवणेन मत्या विज्ञानेनेदं सर्वं विदितम् ||(2.4.5) 

To gain knowledge of Aatma (Spiritual knowledge), one needs to follow three steps, Shravan, Manan, Nidhidhyasan. 

But, what does Shravan, Manan, Nidhidhyasan mean? Let’s find that out. 

Shravan is either hearing from someone or reading by ourselves. This is the most common step for learning and we already know it. 

Manan is contemplating on what we read. It involves thinking about it, asking questions, re-reading, and doing it over-and-over until we understand the essence behind it. 

Nidhidhyasan is internalizing things we understood. It’s like training our subconscious mind. This could be achieved by meditating on what we understood. This step is a little overwhelming to start with, but with practice, it gets easier. 

In short, gaining knowledge is a lot more than just reading. We need to start with reading (Shravan) and contemplating (Manan), then slowly introduce Meditation (Nidhidhyasan). 

All the Best!