How to be smart in life?

There's no certain way. Being smart is just living life in a way that makes sense. There are no shortcuts.
You learn things as you go along in life and being smart is in making the right choices based on what you have learned.
It is avoiding pitfalls, avoiding common mistakes. It is doing things that are important and valuable. It is being sensible, not being addicted to harmful thoughts and substances. It is having sound and balanced psych, maintaining good physical condition, being able to make friends and keeping them, being reasonable in inter-personal encounters and pursing hobbies that leads to above.
It is having reasonable goals and aspirations and the means to achieve them. It is taking measured risks and recovering from failures.