Now just because it's called the Draw tool doesn't mean you have to know how to draw really well!  If you can draw well, then the Freehand Draw is for you!  I personally use the Single Line most of the time.  If I have a tricky reason to use the Draw tool, I'll most likely use the Point to Point.  But first let me point out the benfits of the Single Line draw.  For the examples I'm using a size 3 width so you can see the line.

A few notes here.  If you want to be able to play with the line/lines separate from the image you are using the line on, then either add a new layer or work with a vector line.  Now, with your Draw tool active, left click and hold that click.  Stretch the line out by moving the mouse, in this case to the right.  When the line is where you want it, how long you want it, let go.  While you are stretching this line out, you have to make sure it's straight and you aren't seeing a jaggy line going on.

Look at the images below.  The first image is the clock before I add a line.  The second image of the clock is how the line looks before I let go of the mouse.  There, it's a nice, straight blue line.  But the fourth clock is not going to turn out so good.  I didn't make sure the line was straight before I let go of it.  And see how the blue line turned out?  This would be good if I wasn't trying to add a straight line to a straight part of an image.  Finally I add a nice, straight brown line to the clock so it will be taller.  And because I have the Antialias turned on, no jaggy edges.

So let's get into the Point to Point Line in the Draw tool.  You left click for each node you lay down.  You can cover a lot of graphic territory using the Point to Point.  You don't need a steady hand or the ability to draw a straight line.  Just follow the line of your graphic and lay down points as you go.  See the next page for more on the Point to Point.