I ask because in any problem where you are asked to graph, you should have two variables.
If it IS y = sqrt(9 - x^2)
you can plot points. Pick values for x, and find the corresponding values of y for each of them.
Of course, you need to recognize that you can't have a negative number under the radical sign, so 9 - x^2 >= 0. That limits the values you can use for x....you can't use any value of x which makes 9 - x^2 < 0.
Tell us what you have done to plot some points. When we can see your work, we can determine where it is that you need assistance.