Look into the meshgrid function to create a grid of your coordinates, make sure the syntax of your formula supports element-wise calculation (e.g. use z=x.^2+y.^2 instead of z=x^2+y^2), and use a function like surf to plot it.
If you get an error, read the documentation first. You can open the documentation by typing doc meshgrid, or you can find it by using your favorite internet search engine. Adding 'Matlab' to the name of your function in the query will usually do the trick.
