Just in case you are not kidding. I remember having this kind of problems as a begginner.
You open a Sketch and use the Line command (button). If you are in a drawing , to draw lines you simply use the line command (which is active in a drawing). You must have the Sketch toolbar active, that's where the line button is.
In a Sketch, a line does not help much. You need a shape of your choice that you will extrude and cut and so on. Good luck.