How I  chose my major
It wasn't easy; I had many options and I  wanted to do everything,  so a teacher  told me that first  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  the one with the least disputed seats. Thus, this was my choice.
Bold = incorrect.
Italics = it may not be incorrect, but consider revising.
A few observations:
 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.
 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'.
 Use simple past here, because you're describing past events.
 I added that conjunction so that your speech would flow more smoothly.
 You can either say '(someone) told me' or '(someone) said to me'; 'someone said me' is incorrect.
 You're talking about an event that was a possibility in the past. Therefore, you should use a 'would + verb' structure.
 I would not use the pronoun 'whose' when the subject is not a person or supposed to be a person. Therefore, I rephrased it.