After having supper tonight, I had a call with a new friend from Indonisia after having supper tonightesia.

Sounds more natural with the "after having supper tonight" in front.

He's only 20, still a young boy, but he's already working rahther than going to a(a) college.

It'd be better to use "instead of" and not "rather than" since "rather than" suggests that he'd prefer one thing (working) over the other thing (go to college)

At first, I thought that it' was due to the cultural difference(s) between Taiwan and Indoniesia, andbut he said that it's actually the same circumstance in both countries.

"But" to show the contradiction (you thought it was due to the cultural difference, but it wasn't).

There's another thing that is cool to me; he was once an atheist, but he converted to Christianity after he met heris girlfriend, and this is special to me. / There's another thing that I find cool...

Having "to me" twice at the end of both phrases makes it sound too repetitious

I believe that most people have the same religion their family has originally; this situation didoesn't happen usually happen. / I believe that most people have the same religion as their family... /I believe that most people keep their original religion...

I had a call with a new friend fromAfter dinner tonight, I spoke on the phone with a new Indoniesian after having supper tonightcquaintance.

He' is only 20, years old, so he is still a young boy, buman, yet he' is already working rahther than going to aattending college.

He even talked about marriage with his girlfand his girlfriend even discussed getting marriend.¶

It shocked me because most people don't get married that early nowadays.Being that few individuals nowadays get married at such a young age, I was astonished.¶

At first, I thought that it's dueI had initially attributed it to the cultural differences between Taiwan and Indoniesia, andbut he said that it's actually the same circumstancethe situation is identical in both countrienations.¶

There's another thing that cool to me; he was oncAnother interesting fact is that he used to be an atheist, but he convert to Christianity after he met her girlfriend and this is special to mgnostic before meeting her partner, which is why I find it so remarkable.¶

I believeSince this situation wasn't typical, I think that most people havpractice the same religion as their family has originally; this situation didn't happen usually.ies.¶

He's only 20,still a young boy, but he's already working rahther than going to a college.

He even talked about marriage withying his girlfriend.

I think your version is still correct, although I would have used 'marriage to his girlfriend', since 'with' implies that he was talking to his girlfriend at the time when you and him were talking.

At first, I thought that it's due to the cultural difference between Taiwan and Indoniesia, and he said that it's actually the same circumstance in both countries.

Here 'circumstance' is redundant.

There's another thing that is cool to me; he was once a atheist, but he convert to Christianity after he met heris girlfriend and this is special to me.

I believe that most people have the same religion their family has originally; this situation didoesn't happen usually.

Alternative: 'this situation is unusual'.
I believe that you have used the semicolon correctly here - so well done