The amplitude is half the difference between the max and the min. Your max is 0, your min is -2, so the amplitude is 1. We're going to transform the standard cosine graph, so we have to keep in mind that the standard cosine graph "starts" (at x=0) with a maximum. Your graph "starts" with a minimum, so we have to flip the graph by making the amplitude negative (this is exactly what you were asking about when "a", the amplitude, is negative--doing this makes a max a min and vice versa). Now shift the graph down so that the maximum is 0--do this by making d=-1 (because otherwise the maximum is 1, so you need to drop the graph by 1 unit). This will give you y=-cos(x)-1.
To see the relation between this and the transformed sine graph version, you need only to know that sin(x)=cos(pi/2 -x) (or 90-x if you like degree measures).
Oh, and x is a variable--it doesn't take on any one specific value like a and d can. To generate the graph, x ranges over all real numbers!