Knowing how to draw things requires a lot of work; a lot of studying. If you want to know what an apple looks like, you need to do as many drawings of apples, from as many different angles as possible. More importantly, you need to draw real life apples - either from your kitchen or from photos... DO NOT USE OTHER PEOPLE'S DRAWINGS as references, or you run the risk of inheritting their mistakes. If you want to know how to draw skulls, fill page after page with quick sketches of skulls. If you want to know how to draw roses, fill page after page with quick sketches of roses. That's how you begin to fill your head with what things look like.
Practice drawing quickly - no more than a couple of minutes per sketch. Don't think about what you're doing, just do it. Drawing quickly burns the shape of the image into your brain without slowing down progress by thinking about it too much. Also, make your quick sketches messy and overlapping one and other. Spend no time thinking about making them perfect or clean. The point is to get your hands dirty and experience the shapes of the things you are sketching. Imagine that you are allowing your HAND to learn how do draw, not your brain.