Basically you use a DC A2D, and convert the AC to DC using a precision rectifier circuit - you then calibrate it for correct reading on a sinewave. If you want a true RMS reading, then you can buy specific (fairly expensive!) chips that will do that for you.
For a crude meter, which presumably is all you need? - you could use an attenuator followed by a simple bridge rectifier with a small capacitor, feeding the analogue input of a PIC (or other micro-controller).