I have a best friend in America, butand he is Vietnamese.

He gives me a lot of good advice tofor learning English.

Your sentence is grammatically correct. But this is better, in my opinion.

I am always grateful for that friend.

I am learning English to communicate as ato native speakers.

I have to change my mthe way I thindk.

"Changing [a] mind" means to have a different opinion/belief from before.
Example: You think A is true. You learn that A is false and that B is instead true. Thus, you've changed your mind.
"To change the way you think" means how you think about things and doesn't necessarily result in changing opinions/beliefs

I have a best friend in America, but and he is Vietnamese.

"But" would tend to mean that it's a bad thing. I'm guessing you don't intend to mean that him being Vietnamese in America is a bad thing.
"And" means that he is Vietnamese and is in the United States

If I am fluent in English, I will learn many good things, make many friends around the world, and have new chances in my life.

Alternate: You can use "opportunities" instead of chances.

I am always grateful for thatmy friend.