Pull out a piece of graph paper, which will help you draw your lines exactly horizontally and vertically.
Draw a line, with a straightedge or ruler, going horizontally so that it follows one of the horizontal lines of the graph paper.
Draw points at the end of each line. These points are called endpoints and signify that this line is now a segment.
Take the straightedge and line it up so it is intersecting with the horizontal line you just drew. Line the straightedge up against one of the vertical lines of the graph paper.
Draw a line against the straightedge so that it goes through the horizontal line. The line you just drew is now going vertical.
Draw points at the end of each of the vertical line to signify that this is a line segment.