I read in the Srimad Bhagvatam that Lord Krishna is the creator of everything that exists including demigods. However how did Krishna come into existence. I know that He always existed before but yet there should be a time when He also got into creation. Kindly explain.

It is explained in Katha Upanishad, nityo nityanam cetanas cetananam which means that there are 2 eternals, one is the eternal living entity and the other is eternal Supreme Lord. In the material world we do not have experience of any object which does not have origin. Every object in the material world has beginning, middle and an end.

However spiritual objects are opposite to material objects in the sense that they do not have beginning and an end. They exist eternally. Therefore when we explain God has no beginning that means we can not put a particular date at which He came into existence.

By definition He is the beginning of everything,
ishvarah paramah krishnah
sac-cid-ananda vigrahah
anadir adir govindah
sarva karanam karanam
He is the cause of all causes. If He had some cause, then that cause would have been the superior to Him. This is little difficult to understand in the beginning, however it is possible to understand it by performing devotional service to Lord Krishna.