Why was Thor in a cage and how did he get there?

Question

At start of Thor Ragnarok, Thor is doing the exposition and says something like:

You may be wondering how I got here, I was figuring out these Infinity stones...

(I know it's not exactly what he said, but it was simillar.)

I didn't really get from the story how and why did he really get in the position he's in, imprisoned by Surtur.

Answer 1

The answer is in the words of Thor himself:

"Oh, no! Thor's in a cage.
How did this happen?"
Well, sometimes you have to get captured
just to get a straight answer
out of somebody. - Source

He is having visions of Asgard's destruction :

Because I've been having these terrible dreams of late. Asgard up in flames, falling to ruins.
And you, Surtur, are at the center of all of
them. Then you have seen Ragnarok, the fall of Asgard.
- The great prophecy...

So, as he says, he was there to get his answers and to cancel the upcoming destruction of Asgard. He made it easy to become a prisoner of Surtur on purpose to get his answers.

So he tricks Surtur into telling how Ragnarok can happen and he just gets out of the chains to stop it.

It's a different issue that Ragnarok still happen and Thor made it happen.