First you need to learn numbers. When you are confident with numbers you can easily say the time.

9:15 – девять пятнадцать (nine fifteen)
12:30 – двенадцать тридцать
11:45 – одиннадцать сорок пять
15:00 – пятнадцать ноль ноль
20:20 – двадцать двадцать

You may also use the word “час” (o’clock). But there is one tricky thing you should remember when you use this word – it changes its ending. The ending depends on the number it’s following. If there is 1, the word stays the same (1 час), if there is 2, 3 or 4,  the ending is -а (2 часа). For numbers 5 and bigger the ending is -ов (5 часов).

But after the 20 the ending depends on the last number.

For example:

Сейчас 21 час – it is 9 pm.

Сейчас 22 часа – it is 22 o’clock (10 pm).

Я жду 25 часов – I have been waiting for 25 hours.

Some useful phrases about time

a.m = утра

p.m = вечера

1. Сколько времени? –  What time is it?
2. Во сколько у нас урок? – At what time do we have  a lesson?
3. Я встаю в 6 (шесть) часов утра. – I wake up at 6 a.m.
4. Я ужинаю в 6 часов вечера. – I have dinner at 6 p.m.