In order to learn how to do this, you must know how to: say the numbers 1-29, and how to speak correct grammar
Time is one of the weirdest and confusing subjects (for me) in español. There a lot of twist and turns in it.
If the time is anywhere BEFORE THE 30 MIN. MARK (and it's not one o'clock), than you would say Son las, dos, tres, cuatro(etc.).
If the time is one o'clock, then you say Es la una.
Say the time is 4:20. You would say son las cuatro y veinte. If the time is 2:30, you would say son las dos y media. If the time is 4:40 or 4:55, you'd say cuatro menos veinte or cuatro menos cince. You do this since your subtracting the min. that it would take to get to 60.
There is a chart for reference and a link below to help you further understand.