To make a straight line:
1. Select the Brush Tool (B).
2. Click on the canvas where you want to start drawing the line.
3. Hold Shift and click at the point where you want to end the line, or if you want to draw vertical or horizontal line just hold down Shift and drag whithin the image.

*To adjust the width of the line Control-click (Right-click) and use the slider to set the desired value.