Do you not understand just small parts of the passage (e.g. a hard word, or some small part of the sentence), or do you not understand the overall meaning of the passage? I'm going to guess you don't understand the passage overall -- we call this skill understanding the big picture.
If you use a customize product like PrepScholar New SAT prep, we will actually find out what the issue is, and we have lots of drills to fix your big-picture passage issues. That's exactly the use case of our program.
Overall, as a fundamental skill, the best way to build up to this level of reading comprehension is to read more and more difficult books or short stories. For example, take a look at this list:
and for short passages go here:
Start with easy reading, but make sure you know 100% of it. All the words, and every single possible explicit and implicit (in other words, hidden) meaning there. Who was the author? What was the point BEYOND what was written down?
One of the best ways to improve on this is practice/