Try taking sections of it. If you plug in different values of , what do you get in the plane? Are there values of that are not allowed?
Do you know what is z1=x^2+y^2. It is paraboloid.
Its sections x^2+y^2=k or x^2+y^2=Sqrt[k]^2 are circles with radius Sqrt[k].
So first draw the paraboloid. Then reflect it around the z-axis (just turn it upside down), and finally move it for 1 unit up in the z-axis.
P.S The graph that you need will look something like this: