The slope-intercept form of linear equations is usually used for graphing. In this form, the equation will be:
y = mx + b, where m is the slope (amount the line rises divide by how far it goes to the right) and b is the y-intercept (value of y when x = 0).
In your equation, m = -1 and b = 0.
Since the y-intercept is 0, the line passes through the point (0,0).
Using the slope, we can locate a second point on the line. Since the slope is -1, we need to go down 1 place for each 1 place we go to the right. This brings us to the point (1,-1).
With two points plotted, you can now draw a line through them to graph the line.