Wander around by yourself. Find a work of art that interests you, moves you, intrigues you, disturbs you, or confuses you.
When you write about a painting, consider much more than the objects represented in the painting. Consider the texture of the paint, the frame, the sources of light (both internal to the painting and external, such as the tract lights and skylights in the room), the composition with background and foreground, the relationship of one painting to another, the relationship of objects and lines in the painting to other objects or lines in the painting, the kind of paint used, the visible brush strokes, the way the image looks up close and from far off, the "off" qualities in the painting, the kind of material painted on (canvas, wood, paper, etc.) the style of painting (abstract expressionist, impressionist, neo-classical, etc.), the stories associated with the painting (Susannah and the Elders, for instance), and whatever else helps you write a poem. You may wish to have your poem imitate some of the aesthetics of the painting. In other words, if your painting is impressionistic, you may wish to focus on colors and movement and overall impressions rather than microscopically rendered images. Or you may choose to write a poem which has an aesthetic counterpoint to the painting you choose.
When you write about a piece of sculpture, consider the material (bronze or marble), the traces of hand prints or chisel marks, the balance or imbalance of the piece, the base upon which it sets, the relationship of it to the pieces beside it, the color of the material, the way it reflects light, the emotive quality (tone) of the piece, the "off" qualities that give it unique character, the expression of the eyes or face, the tension in the muscles, the school to which it belongs (abstract, modern, post-modern, etc.)