Once you choose your medium you will then go in search of subjects. The subject that speaks to you is what you should choose to shoot. If a tree and the knots it’s formed interest you, you will want to check the lighting of the area. Deciding which angle to shoot from will also make the decision on the subject. The lighting may not be right for the subject you have chosen and the other side of the subject may not yield the best picture. Your choice of subjects can also change from day to day. Today might be a day where you see the world from a more abstract point of view, where tomorrow may be a day where you see the beauty in objects or nature. The resulting shots will be vastly different and reflect your creativity on that day.
To choose a subject you will need a good eye for detail and observation. Often the best subject is not the one you can see with a plain eye. Have you ever looked at a tree and found a spider web hiding in the leaves? If you look closer you might even find a spider. A spider web can make a great picture not only because of the technique required to have the web show up in your photo with the silky threads, but also the pattern of a spider web. People are fascinated with an organism that can create a symmetrical pattern.