Andrej is the Czech, Slovak and Slovenian form of
Andrew. It is pronounced "Andrey" in these languagues. It is as if you said "
Andre" and added a "y" sound at the end of the word.
Andrzej is the Polish version. This one is pronounced with a very specific "rz" sound that is found in the Polish language as well in Czech even though the two differ a bit. It would sound somewhat like "Andrzhej" I guess.
Andrej