AC voltage measurement upto 230V AC
Look up something called a voltage divider.  That is two resistors that scale down a input voltage linearly.  To get 3.0 V from 230 V, for example, you need one that scales the input down by about 77.

If you want to measure the negative parts of the waveform too, then you have to use three resistors.  Such a setup attenuates and adds a offset at the same time.

There are other issues, like power isolation, but those have also been covered extensively here and many other places.