Summarize:

Choose the Ellipse, Rectangle, or Polygon tool from InDesign's Tools panel. Adjust your background shape's size by clicking one of your shape's handles with your Select tool and dragging it until your shape is the correct size. Make sure the object is selected, then open InDesign's Swatches panel and select the Fill button. Choose the color you want to apply from the Swatches panel. Your shape will be filled with that color or gradient. If there are other shapes or objects on the page, click on your background object with your Select tool. From the Object menu, chose Arrange > Send to Back This will ensure that your background object stays behind all other objects on your page.
Select an object. Click and drag your mouse to draw your desired background shape. Fill the shape. Adjust placement of the object. Place your desired objects over your background shape.