In this case you have a linear equation of the form:
y=mx+b
To graph a line we only need to specify two points of the line and then join them.
b the intercept is zero in this case, and the slope of your line is -1/2. Since the intercept is zero, this means that the line crosses the origin, so the point (0,0) is included in the line, to find other poin of the line we just have to replace a value of x and calculate the value of y, for example let's take x equals 2.
Now, we know that the line crosses the points (0,0) and (2,-1), let's graph them.