A separate letter Y (i griega) represents the sound [i] and is translated as the English «and»:
Mamá y papá – Mum and dad
In other positions, the letter Y is pronounced as the English semivowel [y], though more stressed, like [j] in Jim:
Nueva York = New York
The double Ll (ele doble) is pronounced the same way:
Me llamo Yolanda [me yamo yolanda] – My name is Yolanda
If you want no know if a word is written with Y or with Ll, you must consult a dictionary.