It's hard to say without an example... There are different ways of doing it, or at least to my memory anyway.
Assuming you know what the x-y axes look like... now lay it flat on the table, and then draw the an axis coming out of the origin pointing directly out of the table, that's your positive z-axis. If you're still confused, go here: Cartesian coordinate system - Wikipedia, the free encyclopedia
* Get a graphing program that does 3-D Graphing... there are free ones on the net.
* Sub in z = 0, see what the x-y curve looks like, draw it on the z=0 plane; then repeat with z = 1, z = 2... etc... until you get a feel of what it looks like I guess...
It'd be easier to explain with an equation: is the equation of the plane...
z = 0, y - x - 0 = 0, this is a line y = x
z = 1, y - x - 1 = 0, this is a line y = x + 1
z = 2, y - x - 2 = 0, this is a line y = x + 2
By drawing these lines, you'll eventually see that this is a flat plane... which isn't very interesting... there are funky graphs out there.
Perhaps giving us the equation, we might be able to tell you.