Shreemad Bhagwad Gita is the most holy scripture written in the history of the world. It occupies a part of the great Mahabharata and teaches the reality of life to a lost an confused mind. It teaches one of the biggest lessons ever : Love is not about obssession and attachment~ If you love wholeheartedly you will never find yourself materialistically attached. It propagates the best words of wisdom ever spoken to anyone, by the Lord Himself imparting knowledge to one of the greatest warrior Arjun.

Here are 10 best things which we can learn from the book :

1. The book teaches Karma Yoga:  it teaches that one must control his actions because the results are not going to be in his control. He should not be afraid of performing actions and leave the rest on future. 
2. It teaches Bhakti Yoga:  it states that a person leads his life through his own beliefs. He may look forward to believe in himself, the God or some other person, the choice is totally his.
3. The life will always be full of battles. It is one way of enriching your soul towards purity. So never shrink in fear and dismal; instead try to stand up again and fight till the very end. This is very well depicted through the battle of Mahabharat.
4. Desires:  As a human being, our desires are bound to come & grow. But we should have control over them to not allow them to consume ourselves. Remember having a desire for something and acknowledging it is natural; but one must make sure not to get caught up in those desires, because anger, lust, hatred all are the fruits of desire. If we act upon them and commit evil deeds, the result will never lead us toward salvation. The soul will have to take rebirth for its purity. Bhagwad Gita clearly states that this body is the cloth which soul wears in order to function and act. If we do something wrong with our actions, speech, thoughts then our soul gets contaminated. 
5. Meditation: We can find mentions of meditation in various parts of Bhagavad Gita. Meditation is the best form of non-physical activity to achieve “Inner Peace” and “Sadhana.” The Gita tells that a human mind which is greatly bent towards making money can never concentrate on meditation and achieve inner peace. Thus, for a mind to be relaxed it should be free from all thoughts; otherwise it would continue to be unstable .
6. Whatever happens is for the good:  The Gita states that one must continue doing his/her duties regardless of how many failures or adversities. There is no need to sulk because there is a cause behind every reason and it happens for the good.
7. Change is permanent and it is the only law of the nature: Mother Nature is the Supreme, and anything that is constant in nature is that it's continuously changing. So there is no need to worry about the results too much,. What you actually need to worry is about performing your actions in a moral way.
8. You carry nothing with you on your Doomsday: Many people who spend their whole lives collecting enough things which are never enough for them; and they die in the end- frustrated, and dejected ...nothing good comes out of this. You must know that you came into this Earth empty-handed and will leave from here the same way. So why give your entire life to abundant materialistic things.
9. There is one end only:  There is one conclusion, one destination, one ocean where all the waves submerge at the end, God. The Gita gave the concept of Atma and Parmatma. 
10. To uphold Dharma or righteousness at all costs:  The GIta is a depiction of the moral and the immoral, the righteous and the non-righteous. So, the single greatest lesson it finally gives is to uphold what is righteous at all costs, God will take care of you if you take care of Dharma. Although the definition of Dharma could be seen in different ways from different eyes, it is important to know that one thing never changes and that is God. So uphold the correct form of Righteousness because Dharma equals to Parmatma. And true knowledge of Dharma comes from acknowledging all of the above points, being kind and empathetic towards each other, surrendering yourself to God and though love.