ok first, put the equation into slope-intercept form (y=mx+b). so get y alone.
x-2y=4 --> -2y=4-x (subtract x from each side) --> y= (1/2)x-2 (divide by -2 on each side)
Next to graph, b is the y-intercept, so in this case -2 is the y-value in which the line intercepts the y-axis. So graph the point (0, -2).
Now, m is the slope, so in this case it is 1/2. Slope is rise over run. So from the point you already graphed, go 1 box up and 2 boxes right. Plot another point there and connect the two points into a line.