Drawing what you see is a skill in itself and realising what you want to draw is about incorporating 4 main methods to see what you can draw and how you approach the subject of drawing from observation and these 4 ways help you build on what you want to achieve.
Visualize - This is one of the easiest things to do, the act of visualising your subject and how it could look on paper, a little glimpse into the future by looking at the scene for solid reference and imagining what your intended drawing would look like, to visualize is to work out the drawing in your head and this is important because it can motivate what you sketch on the paper.
Understand - Understanding the ways in which you can get your drawing down on paper comes down to having some drawing experiences and practising the art of drawing what you want and this takes time to understand how you tackle each and every new sketch.
Copy - Copying what other artists do is nothing new, because writers do it, they look at the structure of sentences and see how other authors approach the art of stringing words together and formatting their text into paragraphs, so with other artists you are in effect, studying how they draw, finding out what tools they use to get their art on paper, so quite often you are an invisible apprentice of your chosen artist and you can learn a great deal of useful information and art techniques from them.
Quick sketch - Quick sketching is something that helps you to process the things you see to draw and do it fast, and this is a great technique to use to draw speedy snap shots of what you can find, because if you train your brain to do this daily or even weekly then this skill will build up and be one of your best assets to use in your artistic projects.