Sulaiman, are you asking how to measure the voltage of an AC signal? If your AC signal is +/-2V, you need to move this signal into the range of 0-5V for the adc0804. You can level shift with an op-amp, or use resistors to bias the adc0804 input to half VCC (2.5v) and use a capacitor to AC couple the AC signal. This gets you AC signal in the range of 0-5V. Then, how do you measure this voltage? Since it is AC, you need to know the frequency - lets assume 50Hz. You need to sample this signal for a number of times per AC period. You can test for the highest reading which is the peak voltage or you can calculate the RMS of the sampled values to give a true RMS value. To calculate this you square each sample and calculate the sum of all samples , divide by the number of samples then take the square root of this. The RMS value is what you're left with. RMS (root of the mean squares).

This should be a bit of a challenge in assembler!