Mrs Retro here - there are no hard and fast rules. It all depends where you live and where you come from, your background and the local dialect.
In general, if you have a word that contains a "c" or a "z" followed by an "e" or an "i" it is a soft sound ...... as in "th".
However, in certain regions like Andalucia they talk this way too with the letter "s". The letter "S" is said this way: "th". They call it "hablan con la "C" (thay). It is just the local accent, same as we have Midlands' accents here or the Geordie accent. Not everyone 'habla con la "c"' though. It really depends on your upbringing and piers. For example an Andalucian might say they are from Thevilla and that they were going to eat an enthalada (salad).