/j/ is a voiced consonant sound.
You make the /j/ sound by relaxing the tip of the tongue behind your lower front teeth,...
...and feeling the sides of your tongue on your molars or side teeth like this: /j/, /j/.
A common error with the /j/ sound is turning it into the /dʒ/ sound by bringing the blade of the tongue up.
So, a word like "yolk" ends up sounding like "joke".
Remember: for the /j/ sound, the front of the tongue is always relaxed down.
Notice the difference between /j/ and /dʒ/ as you try it with me and watch my tongue: /j/, /dʒ/
Let's try some sentences together contrasting the two sounds.
I have no use for that juice.
She has yet to travel by jet.
This year, the crowd decided to jeer.
Be sure to put this into practice on EnglishCentral and good luck!