What should I do before I start learning a major?

Before you start studying a science, that is, a science, try to read an overview of its history first. When you are exposed to a new concept, read first about the circumstances in which it emerged and the history of the characters who helped develop it. The English Wikipedia is an excellent starting point 
, and often enough times.

Contrary to what the way sciences are usually taught, different sciences did not emerge on their own. She didn't suddenly appear either.

There is always a context. A frame inside which everything related to the science in question is lined up. Exposing to this framework from the start dramatically reduces the time needed to understand and learn.

Most importantly, knowledge of the history of science changes one’s view of the very process of “study”. It completely changes it. Instead of studying being a very boring activity, it will turn into a "connection" of sorts on a personal level.

There is a huge difference between seeing "Euler" as an equation and seeing him as someone you know. There is a huge difference between seeing Tesla as a unit of measure and seeing him as someone you know.

The history of any science is closer to a novel than to anything else. You have a story whose events take place in different places and at different times..main characters and secondary characters from different social, cultural and political backgrounds..a complex network of relationships in which clusters and collisions are formed .. plot twists .. events that escalate to reach a climax, then the crisis will follow with a solution. The moment you think the story is over, new crises arise, and then the story takes a completely different turn.

No endings. The end is always open. 

No fixed personalities. There are always new characters join the story. You might even join it.

Concepts, formulas, techniques, rules and laws, facts ... all that is usually taught are the chapter headings, while the events within them are neglected. The events of the story have its history. It is what answers you about when, how and why those things that you are asked to know and memorize appeared.

Try to make this a habit.