It's funny, I stumbled into this whole world backwards. My gateway was the 'Sword Art Online' anime, and I only later discovered it was based on a series of books called light novels. That's the core difference right there: a light novel is, fundamentally, a book. It's prose with occasional illustrations, usually one every few chapters. They're designed for a quick read, often with shorter paragraphs and dialogue-heavy sections. The 'light' part refers more to the reading experience than the content—some get incredibly complex.
Manga, on the other hand, is a comic. Your entire narrative is told through sequential art with speech bubbles and sound effects. The pacing is completely different; you absorb a scene visually in a manga, whereas a light novel builds it in your mind through text. For a new reader, I'd say pick based on your preferred medium. If you love getting lost in descriptive worlds and internal monologues, try a light novel. If you're more visual and enjoy the immediate impact of artwork driving the story, start with manga. A lot of series, like 'That Time I Got Reincarnated as a Slime', exist in both formats, so you can often choose your entry point.