I'd strongly suggest that you start reading the getting started tutorials. This is a very basic question, although I honestly have no idea what it is that you really want to do here. If I had to guess, you need to learn how to write a function.
Using a four quadrant atan, this would do what you want.
theta = @(kx,ky) = atan2(ky,kx);
Or do this:
theta = @(kx,ky) = atan(ky/kx);