The pen tool is great for drawing lines because it's very flexible and easy to use. It can be also used to fill the shape you created with the color of your choosing.
Before using the pen tool make sure that you set the settings accordingly:
To create a straight line simply select the tool and press the left mouse button where you want your line to begin and where to end.
To create a curved line you create a straight line, then add an anchor point and move it while holding CTRL.
To create a curved line that curves up and then down proceed as above, except that you move the handles instead of the anchor point.
To actually draw the line/fill the shape simply click the right mouse button and select stroke path/fill path. Width of the line is determined by the stroke setting of the brush.