Q: Where are you from ?

A: I'm originally from a rural part of Saga prefecture.

After that, I moved to Kumamoto cCity to go to university.

I had been studyingstudied there for six years.

Then,Now I've been living in this city for 15 years.

or, even better in the US:

I've been living in this city now for 15 years.

A: I'm originally from a rural part of the Saga prefecture.

You need the 'a', but you can drop the added 'the' if the person is expected to already be familiar with Saga prefecture.

After that, I moved to Kumamoto city to go to university.

In American English it's 'college' instead of university.

I had been studyingstudied there for six years.

You're using *past perfect* tense here but nothing happens after the six years.

When you use past perfect, the next thing needs to happen in the past:
I had been studying there for six years [<then> I moved to a new city].

You moved to a new city, but the time you moved to the new city was in the past.

TSince then, I've been in this city for 15 years.

This sentence started in the past and goes into the future so you need to write it in the *past continuous* tense.

A: I'm originally from the rural part of Saga prefecture.

I lived there until I wasturned 18.

After that,That's when I moved to Kumamoto city to go to universitycollege.

I had been studyingstudied there for six years.

TSince then, I've been living in thise city for another 15 years.

A: I'm originally from a rural part of Saga prefecture.

After that, I moved to Kumamoto cCity to go to university.

"move" becomes past tense -> moved

I had been studyingstudied there for six years.

Then, I've been in this city for 15 years.

"Then, I've lived in this city for 15 years." is also OK!