What shockinglyhockley meant was that there's another post about this topic :-)
That said, it depends what you want to do. Easiest way would be to create a square-wave that's nothing more than a counter that counts at a specified frequency and inverts the output signal, which you can directly (given the proper precautions!) hook up to a speaker. Alternatively, use a ROM and use the same counter to index into said ROM and pull samples out which you can then output on a DAC or PWM generator.
-Mux
