Speaking ofI want to talk/say more about how serious I am about learning English, I want to say more about myself in this aspect.

Less wordy this way

When I was a child, I had a weak body, so that I often got sick.

You know, at that time, there were no advanced technologies andnor agricultural machines for farm work.

No advanced technologies and no agricultural machines, so use "nor"

So, people did farm work only with their hands and legs. / So, people did farm work manually.

"Manually" means done by hand instead of done by machine

Knowing that I couldn't do farming, I madecame up with the idea that I must study hard to get an opportunity to work for the government. / ...I decided that I must study hard...

When I was in school, I was always one of the top three students in my class, -- even in the grade -- in the school.

If I went to the high school, I needed to take the university entrance examination after three years (of learning).

Not going to study in a high school and aor university is my biggest regret in my entire life / my life's biggest regret.

"Study" is already implied by high school and university so you don't need it, plus it's more natural this way

Whenever I go to a city, visiting the famous university in the city is my regular routine, whichwhich represents my dreams that cannot come true is my regular routine. / Whenever I go to a city, my regular routine is to visit the famous university in the city that represents my dreams that cannot come true.

Putting the phrase "which represents my dreams that cannot come true" makes it seem like the entire action of "visiting the famous university in the city" is what is representing your dreams, but it should just be the university that is representing your dreams.

I only remembered a few words, such as thanks, yes, and no.

We usually add a conjunction at the end of a list. If we leave it off, it's usually for emphasis

I will keep it up and never slack off / rest.

When I was a child, I had a weak body, so that I often got sick.

"so that" usually implies you did something with the intent of achieving a certain goal; ex. I studied so that I would do well on the test.

It needed farmersrequired farmers to be very strong. -OR- Farmers needed to be very strong.

It sounds more natural this way. "It needed" would sound normal if it was a person needing them to be strong, but not the work itself.

Knowing that I couldn't do farming, I madecame up with the idea that I must study hard to get an opportunity to work for the government.

When I was in school, I was always one of the top three students in my class, even in the grade in thegrade, and even school.

In the high school entrance examination, I gotwas the number one student in the school among two hundred students.

"got" here only works if you obtained something specific, ie., if you got a 'Number One Student' award. Otherwise, the number one student is what you were.

With the good performance, I could learn ingo to the only provincial key high school in my city, or learn ingo to a specialized secondary school.

'Learn' requires an object, so you could learn science at a specialized school, but you can't just say that you learn at a specialized school.

The only thing that I needed to do before I started working for the government was to study inat the school for three years.

Back then, middle school students who were from the countryside all chose to study inat a specialized secondary school if they performed very well in the high school entrance examination.