There are several methods to achieve simplicity in a photograph. The most obvious (and easiest) is to place the subject against a neutral background such as a backdrop or the sky. Backgrounds can be entirely neutral, like a solid backdrop or a cloudless sky; or they can complement the image, like a starfish on the sand.
The more technical method of achieving simplicity involves focusing on the subject while ensuring the background is unfocused. Macro settings on digital cameras tend to do this automatically, as they have a narrow depth of field to begin with; the same effect can be achieved with manual adjustment. The picture taken to represent simplicity should have a clear reason that object was chosen. It should be clearly evident that that's the reason for taking the picture. All unrelated topics shouldn't be present. This relates to Shallow Depth of field as well, which blurs out the background and focuses on what is on the middle