A decreasing graph can be obtained with a line that has negative slope. That is one of the simplest function you can have.
For example:
f(x) = - 3 x +2
or using the vertical variable "y" the form:
y = -x + 5
Either function will render a decreasing graph, as shown in the image below:
where the function y = - 3 x + 5 was plotted