To make Italian Sodas, you need plain fizzy water, though some people use Soda Water (aka Club Soda, the kind with bicarbonate of soda in it) and either don’t notice any appreciable taste impact, or like it.
For the flavouring, you can buy the syrups at coffee shops, specialty food stores or Italian delis. Well-known brands include Torani. Some people make their own soda syrups at home, by boiling sugar, water and flavourings. Some people think Italian soda syrups are actually an American invention, because they don’t remember seeing them in Italy, but they are available in Southern Italy, in places such as Sicily.
You put some syrup in a glass, fill glass with ice, then add the fizzy water. Generally, you put only about 1/4 inch (1/2 cm) in your glass. The back of bottles will give the recommended proportions, and typically say 1 part syrup to 10 parts fizzy water. Some people prefer a higher syrup ration, of 1 part syrup to 4 parts fizzy water. If you use more syrup, in the proportions of half syrup, half water, the drink is called a “Cremosa.”
Italian Sodas are sold in Italy from a “chiosco” (aka kiosque, called “cioscu” in Sicilian.) People often stop at the end of an evening to have one on the way home from work. They call such a mixed drink a “bibita.” If you add a lemon wedge, becomes “al limone.”
A plain soda with no syrup in Sicily is “seltz al limone con sale” (carbonated water with a lemon wedge and a dash of salt.)
Sometimes Italians will use mineral water instead, either plain or fizzy mineral water, and they may make a batch of flavoured soda up by the jugful at home instead of by the glass.