ANSWER:

What you have heard is incorrect 

You can start the ghusl without cleaning the impurity as it will be cleaned while you do ghusl

QUESTION: I have heard that inorder to make fard ghusl it is obligatory to be free from impurities(like urine, wadiy) from body before starting ghusl. My question is, is it obligatory to remove najasah (like urine) from body before inorder to start and perform fard ghusl? Or we can start the fard ghusl even if urine is present on body and we clean within inside of fard ghusl?