What is the most important part of financial planning for an individual, with arguments to prove your opinion?
Since you are a young person, I will talk about what I think is the most important part of financial planning for a person of roughly your age. In the United States, at least, the most important part of financial planning is getting yourself a good credit rating.
The reason that this is so important is that it will affect your ability to borrow in the future. It will also affect the sorts of interest rates that you will qualify for. If you can establish a good credit rating from the start, you will have many more opportunities to borrow money at relatively low rates. This will be very good for you when you get to the point where you need to finance the purchase of a house or a car.