Lots of people here recommending starting with The Hobbit (book) for "world building" and backstory reasons. I don't agree for a couple reasons.

First, its a totally different kind of book--light children's fantasy compared with high fantasy in an epic/mythical world. I'm not sure liking the former would necessarily mean liking the latter. (I, for one, didn't really care for the book as a child or an adult. I found it boring.). There are different emphases, different stylistic choices, and characters/races are presented in different ways.

Second, Lord of the Rings contains enough exposition and dialogue to give a first-time reader a sense of what happened in the past. If one really wishes to know the specifics, they can read The Hobbit as a "prequel." In fact I'd argue that it works better that way: The Hobbit reads better as Bilbo's subjective in-world account of what happens whereas Lord of the Rings reads better as an objective narrative by an omniscient storyteller.

I'd say start with the books and if you aren't grabbed, try the films. You might feel compelled to go back and try the books again. I'll let others argue for or against the extended editions but I'd say they're the better adaptations.