On the Airplane

I'm traveling with a friend to Boracay. (Sounds natural)

Since it was chosen as a best beach in Asia a few years ago and it's high season now, there are so many tourists. (Again, does not sound too natural)

Since it was chosen as the best beach in Asia a few years ago, it's high season now. Because of this, there are a lot of tourists.
- When saying "the best", it means that it is number 1. The top of everything else. you can also say "One of the best beaches", meaning that it is very high up there, but I am not sure if it is the best beach of every other beach.

By the way, when we were on the airplane, I went to the bathroom.

So, when we were on the airplane, I went to the bathroom

"By the way" is usually used when you want to say something after a conversation has happened, or when a conversation is about to end. "So" is used when there is a small change in your story, or if you are continuing your story.

There was a speaker and I listened to man's voice suddenly.

Suddenly, there was a man's voice over the speaker, and so I listened.

He spoke very fast and I didn't catch what he said.
I thought someone was talking to me and I might have done something wrong there.

I thought someone was talking to me, and that I might have done something wrong.

But it was just information for all of us.
Afterward, I talked to my friend about that and she was laughing.

Afterwards, I told my friend about it and made her laugh.

This is a lot of information, but I hope this helps with your studies!

英語がちょっと難しいですね。頑張ったください！