I'm going to tell you about half notes as they relate to 4/4, or common, time. In common time, there are four quarter note beats per measure. If you turn on your metronome and play one note per click, you are playing quarter notes.
Since a half note lasts for twice as long as a quarter note, then you should play one note every two beats of the metronome in order to play half notes. There are two half notes in a measure of 4/4 time.
For the next lesson, you can practice couting quarter notes .