Whatever the climax of your story, it should generally affect the protagonist in a deep and meaningful way. It should be a pivot point, a point where everything changes for your characters. What this means will differ based on the context of your story. Going back to Lord of the Rings, Frodo not only almost dies at Mount Doom, but he is forever changed.
So how do you craft this climax? Well that will depend entirely on your story and what the point of your novel was. It's a good idea to have your climax in mind before you even begin writing your story. That way you can guide your story towards your climax. You can put hints and allusions throughout your story that almost foreshadow your climax. Your climax might even be clear from the beginning. Sticking with the Lord of the Rings example, you know quite early on what the climax of the entire story is. The Council of Elrond makes it clear that Frodo is going to take the rings to Mount Doom and destroy it. That's the climax. It's revealed early on. You know where the story's going.
That can be a blessing. It gives your readers something to look forward to. They know what to expect, know where they're going, and just need to see how you're going to take them there. But this isn't the only way to get to your climax. You can be more subtle, you can even outright mislead your readers if that's what serves the story you're trying to tell. If you're writing a mystery, for example, you can often surprise your readers with the climax. Perhaps the perpetrator isn't who anyone thought. Or is exactly who everything thought it could be. There are so many ways to approach the climax of your story.
The really important part of the climax is that it be satisfying. It should be the payoff for sticking with the story or an entire novel. It should also wrap up the vast majority of loose ends. It doesn't have to address them all. That's what the denouement is for.
