To create a structural member in Solidworks, you can either create a sketch or use the Rectangle tool to draw a rectangle that will be the size of your structural member.
To create a sketch, go to the Sketch tab and select Line from the Draw panel. Then, click on the Rectangle tool in Draw toolbar at the bottom of screen. Select Line from Line menu. Pick up your drawing tool and start drawing lines on your sketch until you get something that looks like this:
To finish it off, select Line from Line menu again and pick up your drawing tool to erase what you just drew.