The most important thing about abstract photography to think of when finding and composing images is to remember to minimalize if not eliminate context. A photograph of a road can be abstract if you don't have buildings to give it scale or perspective. A rusty car becomes abstract when you photograph it such that the rust becomes a pattern, or the shapes and form of the vehicle is the subject.

Or do photograms or still lives - look at Ruth Bernhard and Laszlo Moholy-Nagy for examples.