To ensure you are appropriately counting along with the rhythm of the song, listen to the beats of the song and ensure that you are counting in time. For example, count four quarter notes in a 4/4 song.
Identify any triplets. These can be noted in several ways. Most commonly, the triplet will be notated with a bar above the notes and rests -- with the number three above the bar -- to indicate that the three notes are played as a triplet, and thus takes up the space of two normal notes.
Identify the time signature for the song. In the most common time signatures, a quarter note is considered a single beat. Thus, two eighth notes will make up one quarter note, and thus one beat.
To count a triplet of eighth notes, you will need to use a system that counts each triplet as a single beat. For example, a common way to count triplets of eighth notes is to count "one and a, two and a."
Whenever the musician says one of those words, he will play a note in the triplet. By beginning each sequence of three notes with a number, he is able to keep track of which beat the musician is currently playing. If the musician counts a triplet as "three and a," he is playing the third beat in the measure and plays three notes, one on each word.