To start, go to the "Mechanical Design" and then to the "Generative Sheetmetal Design" workbench. Click on the icon and specify the correct parameters of the sheet that you are going to work with: "Thickness", "Default Bend Radius" and "Bend Extremities". The "Sheet Metal Parameters" feature should be added in the specification tree.

If you want to see step by step how to do it, please follow the document below.

 Getting Started with Sheetmetal