What were the lessons that Lord Krishna taught Arjuna during Mahabharata?

Lord Krishna's teachings to Arjuna highlighted that all wars first exist in the mind. And thus winning in the mind is the first step to winning on the battlefield and in real life. Here are some of them

Think with a calm mind: Lord Krishna says: Undoubtedly, O Arjuna, the mind is restless and difficult to restrain, but it is subdued by any constant vigorous spiritual practice -- such as meditation -- with perseverance, and by detachment.

The first step to gaining clarity on any situation is developing a clear, calm and collected mind. This takes a lot of effort. One way is meditation, another is by distancing yourself from the situation - not physically but mentally - where you look at it as an outsider and have a bird's eye view of it.

Give up on results: You have control over doing your respective duty only, but no control or claim over the results. The fruits of work should not be your motive, and you should never be inactive.

Often referred to as “nishkam karma”, doing action without expecting reward. Most of our decisions get affected because we wonder about their outcomes and consequences. But when you realize that you have little control over the final outcome and when you don't focus on the gains, your efforts will be filled with more meaning. What's more you will also look at every gain as a bonus and appreciate the rewards more.

Treat everyone equally: A person is considered superior who is impartial towards companions, friends, enemies, neutrals, arbiters, haters, relatives, saints, and sinners.

People make up our lives and it's difficult not to get influenced by them or by our equations with them. But that's exactly what Lord Krishna says. Treat everyone with the same lens of impartiality. A son shouldn't take on his father's business by virtue of being his son, but, because he is an able and competent worker. Likewise just because someone has picked a fight with you, don't write them off for good... the event isn't the person.