The Primary sources of the Krishna story are four scriptures: the Mahabharata, and its adjunct Harivamsapurana, the Visnupurana and the Bhagavata. The story is also met with in other puranas like the Brahmapurana, Padmapurana and Brahmavaivartapurana (Krishnajanma-khanda).
In the Mahabharata, Krishna appears for the first time during the marriage of Draupadi with the Pandavas. Through there are casual references to his earlier life, no details have been given. To make up for this lacuna, as it were, the Harivamsa was added as an adjunct, later. It is a fairly long work of over 16,000 verses and deals with the story of Krishna in great detail in the section called Visnuparva.
The Vishunpurana (6000 verses) is one of the earliest puranas and deals with the story of Krishna in the fifth section (pancamamsa). The Bhagavata (18,000 verses) seems to be an expanded version of the Vishnupurana in that it deals with practically the sane topics, but in much greater detail.