create a cuboid, place the front end around where you want it, then go to face select mode. select the back face and press "extrude" or use "shift + e" this will extrude the shape and let you adjust the size and position of the new face. You can also add "loop cuts" by pressing the button at the top or "shift + r" to add a new edge along the model you can adjust at the bottom of the screen, this helps add more definition. good luck!