Generally, it’s -ise in British English (as in French), and -ize in American English (as in Greek / Latin). There is no difference in meaning, so just choose British or American spelling, and you’re good to go. This does not affect the pronunciation, which is always /z/.
Special note: “Practice” is spelled “practise” by British speakers when it is a verb. Americans write “practice” both as a noun and a verb — this follows the spelling in Latin (and the pronunciation in French). This does not affect the pronunciation, as the S in that word is unvoiced as a noun and a verb.
Some words really do have /s/ as a noun and /z/ as a verb, for example “some advice” (noun) and “to advise” (verb). In this case, the spelling is the same in British and American usage. “House” is always spelled with an “s”, although, the verb ends in /z/ and the noun in /s/.