Immediately after having launched Blender, move the mouse arrow on the cube, right click on it, press the “delete” button and click “delete” on the dialog box. Similarly, move the mouse arrow on the camera, right click on it, press the “delete” button and click “delete” on the dialog box.
Please, note that removing starting cube and camera is possible in the “Object mode” status (see lower horizontal bars), which is the status you will find at the startup of Blender, and not in the “Edit mode” status.
If you wish to save this “clean” scenario so that at the next startup of Blender no camera and cube will appear, then simply CTRL + u and click “save startup file”. You can always restore the initial Blender settings by File -> Load Factory Settings and then CTRL + u.