What is tension? With regard to reading, tension is the single-most effective way to keep a reader turning pages. But it's also one of the hardest things for a writer to pull off.
Focus on micro-tension. If you can keep the reader engaged from moment to moment then, eventually, they will reach the end of the book. Those moments add up.
For example, (since me and the fam just recently re-watched one of our fave movies of all-time, I'm using it) in Jurassic Park's T-Rex scene. You know which one I'm talking about. THE T-Rex scene.
That scene is full of tense moments. Individual moments that keep you on the edge of your seat, but that also contribute to the story as a whole. In each moment, the viewer wonders only about what will happen in that moment. That is what makes it tense. When Lex is so scared she can't even get the flashlight turned off and it keeps blasting light on the T-Rex and drawing its attention, are you at all worried, in that moment, about Dennis Nedry's plan to steal the embryos?
I should hope not. Your main concern is OMG IS SHE GOING TO GET EATEN! You can worry about the other stuff later, once you know she is safe.
When Tim is stuck in the car that's stuck in the tree and they start climbing down and the car starts falling toward them, are you at all worried, in that moment, about any of the dinosaurs that are now roaming free? Not really just yet, because your main concern is OMG ARE THEY GOING TO GET CRUSHED!
Same goes for the kitchen scene with the Raptors later, which is by far the scariest most tense scene in the whole movie. On that point, the reason the kitchen scene works so well in the anxiety department is because the tension is drawn out as taught as it possibly can. It's the least bloody-gory part of the movie yet it was chosen for the ending because it has the most intensity. The worse something is, the closer it should be to the climax.
Even in the not-so-action-filled scenes, there is still tension as we absorb everything that's happened or is going to happen. Tension does not always equal action. It always equals worry, whether the current pace is high or low.
Worry for the moment keeps readers turning pages. But at the same time, you can't just have a bunch of unrelated intense events strung together. Readers will quickly get frustrated with the what-does-this-have-to-do-with-anything? factor. Every event must aid in the outworking of the plot. Look at any of those examples above and you should see what I mean. They're all connected to the theme of the story-- Life will find a way.
This why it is so difficult to master. This is why structure is so important when writing a first draft. And this is why close analyzation of every element in your novel is so important when revising.
Take a page, any page that you've written, and judge it by its own merit. See if you can find specific reasons why the reader should keep reading. Specific qualities that make it tense. On that page alone.
The three main things to look for are:
1. A clear narrative voice.
2. A sense of where you are and why.
3. A reason to care what happens next.
There are so many different variations on how one can effectively portray all three of those on a page. And yes, much of it rides on the individual opinion of the reader.
Number one cannot be taught, in my opinion. Voice is something you learn how to do on your own, with practice. Lots and lots and lots of practice.
Number two involves the writer's use of description. Not too much and not too little. You want the reader to know just enough to avoid confusion. Because the real focus should be on...
Number three, making the reader care what happens next so they'll turn the page. That is where correct tension comes into play. It's a balance of intrigue and conflict, whether that be inward or outward.
Which brings me to another point. On a first page, not every story needs to start out with a slam-bam action scene to be considered tense. The tension from the hinted conflict can be inward. And the bottom line is, as long as the reader has a reason to care about what happens next, they will keep reading.
It really is that simple. Um... No, not really. It's the hardest thing ever. But not impossible to achieve. Read and analyze novels that kept you engaged (how did the author do it?), and keep practicing your technique. You'll get there.