For 7:15, 7:30, and 7:45, you have two main choices: Use fifteen, thirty, or forty-five or "quarter" and "half."
It's seven fifteen. / It's (a) quarter after (past)seven.
It's seven thirty. / It's half past seven.
It's seven forty-five. / It's (a) quarter of 8:00.
For other times, like 7:23 or 7:56, one form is most common.
It's seven twenty-three. (Not: "It's twenty-three after seven." It's possible and correct, but not common. You could use it if you have to be precise for some reason.)