Take a look at Java SimpleDateFormat.
system.debug(Datetime.now().format('hh:mmaa')); // 08:24AM
system.debug(Datetime.now().format('hh:mm:ss aa')); // 08:24:07 AM
system.debug(Datetime.now().format('yyyy-MM-dd')); // 2016-05-31
Note that these are methods on Datetime, so you should not call date() or time() on your Datetime value before doing the formatting, as doing so will change the type to Date or Time respectively, and neither of these types have a format method defined`.