Measuring AC with ADC

HI Guys...I find that there are many threads available on ADC interfacing and then using the same for connection to a DC line.

I also understand the suggestions of dedicated ICs fro RMS to DC extraction or energy metering.

But I want to understand if we could measure/ GET THE digital value through an ADC of an AC signal. In may other forums I have read people suggesting the following, but i could not understand much to start and implement, so thought may be you guys would help me :

1) Measure the signal at zero Crossing: I don't know how can we take the sample, when the signal is at amplitude 0(rise) and amplitude 0 (fall)

2) With appropriate Sampling rate we can measure the AC signal and through a routine of comparing the current reading with previous reading, we can extract the highest among many sample readings: When I tried and implement the same , the results are not satisfactory.

The ADC is 0808. The Uc is 89c51rd2, signal specs freq: 50 Hz, 300mv Peak to Peak, offset : 300 mv with reference voltage as 300 mv DC line in:
Simulation: Proteus. Compier: KEil

If you guys suggest me some other appropriate place to post this query, please direct me for there.. Look forward for your help, suggestions and kind guidance.