Confusing timeline

I just finished the firat book and I am very confused. I should mention I watched the series too.

I was under the impression that the world always repeated and that there was thousands of Dragons before yhe events of the books. But after looking online it says that the book is happening in the third age and that Lews was in the second so there was only one Dragon.

Was there a world before the first age? Is the Dark One always being fought and captured so a new Dragon fights him? Is there going to be one after?

Thank you.