Mt Etna's southern slope is the most accessible and popular gateway for those wanting to ascend the volcano, and the starting point for getting up to the crater area is Rifugio Sapienza (1923m), a small cluster of souvenir shops and bars based around the eponymous mountain refuge. From here there are various options for heading up towards the peak.
The easiest is to take the Funivia dell'Etna up to 2500m and then a minibus to the Torre del Filosfo at 2920m. Alternatively, you can forego the minibus and walk from the upper cable-car station. It's quite a steep 2km walk and you should allow yourself up to four hours to get back in time for the return cable car. Another option is to walk all the way from Rifugio Sapienza, but this is a strenuous climb that will take about four hours (less on the way down). Note that in windy weather the cable-car service is suspended and replaced by a minibus.
There are four craters at the top: Bocca di Nord-Est (northeast crater), Voragine, Bocca Nuova and Cratere Sud-Est (southeast crater). The two you're most likely to see are Cratere Sud-Est, one of the most active, and Bocca Nuova. How close you can get will depend on the level of volcanic activity. If you're hiking without a guide, always err on the side of caution as the dangers around the craters are very real. To the east of the crater area, the Valle del Bove, a massive depression formed after a cone collapsed several thousand years ago, falls away in a precipitous 1000m drop.