Correção the texo: Why I select my major

[1] How I [2] chose my major

It wasn't easy; I had many options and I [3] wanted to do everything, [4] so a teacher [5] told me that first [6] I would have to choose the area. Since I was good at math and physics, I decided to choose a major in some exact science. Then, people said that I should choose a major in something that I liked too. So, I narrowed down my options to Mechatronics, Computer Engineering, Computer Science, Information systems and Electric Engineering. Among those, Information Systems was [7] the one with the least disputed seats. Thus, this was my choice.

Correction key:
Bold = incorrect.
Italics = it may not be incorrect, but consider revising.

A few observations:

[1] Once you're describing a personal experience in its entirety, I would use 'how' rather than 'why'. It's just a matter of personal taste, though.

[2] Since you had already chosen your major by the time you wrote your text, the sentence should be in the past simple. I found 'choose' more adequate to your sentence than 'select'.

[3] Use simple past here, because you're describing past events.

[4] I added that conjunction so that your speech would flow more smoothly.

[5] You can either say '(someone) told me' or '(someone) said to me'; 'someone said me' is incorrect.

[6] You're talking about an event that was a possibility in the past. Therefore, you should use a 'would + verb' structure.

[7] I would not use the pronoun 'whose' when the subject is not a person or supposed to be a person. Therefore, I rephrased it.

Cheers!!