A lot on how you do this depends on the accuracy you need, and the range over which you need a reading.

A good way is to use a decent AC step-down transformer (the very tiny cheap ones don't work very well). Choose one that has the right input voltage, and an output voltage of 12-24V p-p. Use that output to feed a bridge rectifier and voltage divider. Keep the impedance of the voltage divider below 3K ohms (as seen by the PIC) for best accuracy. Put a small (.1 uF) cap on the output of the divider to kill any spikes. Also note that some of the little transformers have an output waveform that is very load-dependent. Use a scope to make sure yours is putting out an undistorted sine wave and choose your voltage divider accordingly.

And while I have the "floor" - I can't understand why everyone isn't moving over to the PIC 18F series. They have much more capabilities, are better in almost every way,
and don't cost much more. ?????