Actually, you have to separate it more than just OT vs. NT. The bible is made up of dozens of individual books, most of which had multiple authors and editors, and many of which were written/compiled over a long period of time.

Obviously there are some events mentioned in some books of the bible that are corroborated by many other sources, such as the fact that Darius was the king of Persia, or that certain parts of the Mediterranean world were subject to Rome in the first century CE. Other books of the bible contain myths that were obviously written as myths without any pretense to historical veracity; they were stories illustrating their points by metaphor, not by factual evidence. And there is poetry, and there are specific genres such as "apocalyptic" that also do not pretend to be historically accurate accounts of events.

"The bible" is a very complicated group of texts that have to be considered in appropriate contexts; it's not a unified whole of a "book" (or two "books").

If you "don't know who to believe," then you need to learn some research skills and critical thinking skills. There are many scholarly books in your library that lay out the various forms of evidence for the historicity (or not) of different accounts of events in the bible. Those scholarly works cite their sources. Reading that kind of analysis is much more useful in "knowing who to believe" than just "hearing various people" (who may not have any expertise in the subject) "give their opinions" (which may have no sources to back them up).

Not to discourage new responses, but there've been a lot of questions and good answers about the Bible's historicity in this subreddit before. I'd recommend you check out a lot of them (not all) collected here in the FAQ.

I also suggest looking at this subreddit: https://www.reddit.com/r/AcademicBiblical/