Say "Es la una" for the one-o'clock hour. For all other hours, say "Son las..." For example, for 3:00 you would say, "Son las tres." This is because one hour is singular and and more than one is plural.
Say "y" after the hour for all times before the half hour. For example, 5:20 would be "Son las cinco y veinte." The clock reading 1:05 is said, "Es la una y cinco."
Subtract from the next hour for times after the half hour, and say "(hour) menos..." For 3:50, you would say "Son las quatro menos diez." The English equivalent of this is "Four minus ten" or "Ten 'til four."
Say "media" for the half-hour. 11:30 is "son las once y media."
Use "de la mañana" for "in the morning," "de la tarde" for "in the afternoon" and "de la noche" for "at night." For example say "Son las seis de la mañana" for 6:00 a.m., and "Son las seis de la tarde." for 6:00 p.m.
Say "la mediodía" for noon, and "la medianoche" for midnight.