I would NOT recommend measuring lethal voltages with the Arduino!
And, I wouldn't try building a regular multimeter with the Arduino. There are special purpose chips for this, but personally I still wouldn't bother building my own meter.
It's tricky and it can be dangerous, especially because of the "exposed ground". A battery-operated meter has a black/reference lead and it's internal ground is "floating". An AC powered meter has an isolated power supply so again there is no "ground". With a regular meter you can reverse the leads and it will work fine (and DC will read simply negative.)
But, if you put voltage on the Arduino's ground there can be a short to through the USB ground or power supply ground and bad things can happen to you, the Arduino, or your computer (if the USB is connected). You could get killed by touching the Arduino's ground!
The input of a meter has to be over-voltage protected. (The Arduino can be damaged by negative voltages or voltages above +5V.) A voltage divider will help but you still need a protection circuit.
For higher AC voltages you can use a diode rectifier and a filter capacitor. (I think my meter at home has a rectifier because it's not good at low AC voltages.) Of course, you need a voltage divider and an over-voltage protection circuit following the rectifier.
For lower AC voltages you can build an op-amp "precision rectifier" circuit. Most meters measure the peak rectified voltage and then multiply by 0.707 to get the RMS. But, that only works with sine waves and there are more expensive "true RMS" meters that actually calculate RMS.
Resistance and current measurements are trickier...