It depends on what your function f is. Your function f will show some relation between x and y. Also you should define range of x and y. Checl the below demo example.
% DEfine x, y
x = linspace(-1,1) ;
y = linspace(-1,1) ;
% Make a grid
[X,Y] = meshgrid(x,y) ;
% define the function
f = X+Y ;
z = NaN(size(f)) ;
z(f<=0) = f(f<=0) ; % make other value NaN
surf(X,Y,z) % plot
shading interp
colorbar
