In Spanish the letter j is pronounced as the "h" in the word "heart". The letter y is pronounced just like the "y" in English . Let's look at a few examples.

j
Soy jardinero.
     I'm a gardener.
Él es el jefe.
     He's the boss.
Mi mejilla está fría.
     My cheek is cold.
carro viejo.
     old car.
Yo juego con el perro.
     I play with the dog.

y
Vamos a la playa.
     Let's go to the beach.
Ellos jugaron fútbol ayer.
     They played soccer yesterday.
Ella es mayor que yo.
     She's older than I.
Yo ayudo.
     I help.

Note: There are only a couple of words with yi, and they are not commonly used.