This is not the best composition but hopefully it will work for the explanation of looking for shapes. Looking for the shapes in your subject probably doesn't come easy for most. When you were a kid it was super easy. We were given toys that were made in basic shapes and we built things with them and fit them inside other things and shapes just made sense.
The biggest problem for beginning plein air painters is what to paint. After you get all the fun gear and pack up to go out and paint you must then decide what to paint. I covered a solution to that problem in a previous post. After you decide on what to paint the problems really start to mount. The light changes, your subject moves, you dropped your paintbrush in the river, so on and so forth. Since you're going to have these obstacles up against you don't want to add the painting experience into the pile of misery. (Honestly, if you've dropped your box of pastels or had your oil painting face plant into a pile of weeds as many times as I have you'll know it's just best to laugh and not be miserable.)
Again, there are many ways to go about this so try things out and adopt whatever method helps you get the job done.