You can start by plugging in values for x, multiply it by -3 and subtract 3 to get the corresponding y value. This will give a table of values like this:
{:("x","y"),(-2,3),(-1,0),(0,-3),(1,-6),(vdots,vdots):}
You can then plot these points. You can plot any point (x,y), by starting from the origin, (0,0). The first number is the x value, which tells how far to move left or right. The second number is the y value, which tells you how far to move up or down.
All you really need is two points though. Connect the dots with a straight line and you have your graph. Notice that when the x values increase by one, the y values decrease by 3. The amount of y change (or "rise") divided by the amount of x change (or "run") is called the slope of the graph, m, and is an important feature to help you understand how "steep" a straight line is.
Here is what that graph will look like when your done.
graph{y=-3x-3}
