You can make a table to start.
Pick random values for x, and then find y.
If x is 0, then y = -3(0) = 0
If x is 1, then y = -3(1) = -3
You could stop there because you only need two points to make a line.
One point is (0, 0). Another point is (1, -3). Connect the dots.
Faster way to do this: graph is in the form y = mx + b, in which m is the slope and b is the y-intercept (in this case the y-intercept is 0, so there is no b value).
For y = -3x, the slope is -3, or -3/1.
Slope is change in y over change in x, or rise over run, so from the origin (since the y-intercept is 0, x=0), go down -3 and to the right 1 and that's the next point. Go down another 3 and right 1 and you have another point.