The video on this page explains how to count eighth note triplets. Unlike quarter, eighth, sixteenth, and thirty-second note sub-divisions that divide time in half - eighth note triplets actually divide quarter notes in three. In other words, one quarter note is equal to three eighth note triplets.
This first example shows a measure of eighth note triplets. As you can see above the note groupings - the second and third notes of each group is counted using the two syllables of the word triplet. The entire measure is to be counted one, trip, let, two, trip, let, three, trip, let, four, trip, let (re-starting at one for each measure).
It's also important to note that like regular eighth notes - these are connected by a single horizontal line at the top of the note stems. However, the little number "3" indicates that they are to be played as "triplets".
This second exercise has a measure of quarter notes leading into a measure of eighth note triplets. Watch the video lesson on this page to see how this is counted and played. Take note of the fact that odd groupings require you to switch lead hands for the one, two, three, and four counts if you are playing these measures with alternating hands.
This third example includes quarter notes, eighth notes, sixteenth notes, and eighth note triplets. Count through it carefully with a metronome to be sure you are staying in time. You can re-watch the video lesson to see this exercise demonstrated an unlimited number of times.