It might look straightforward at first, but to draw straight lines in Photoshop there are a couple of things you can do.
The line tool does what it says but you can't draw lines continuously.

The quickest way to draw straight lines is with the brush tool selected, click on the canvas, tap and hold the shift key and click on the canvas again. Photoshop will draw a straight line between these two points.

The next thing you can do, you can use the pen tool to draw your line(s), rightclick the path, and 'stroke path'.

A similar way to do this, but with a different tool is using the polygonal lasso tool. Just draw your selection and again rightclick and hit 'stroke'. The context menu here looks a little different but does essentially the same thing.