In our common observation that water flows from a place at a higher level to a place at a lower level or potential irrespective of amount of water at the two levels . Heat flows from a body at a higher temperature to a body at a lower temperature irrespective of the amount of heat contained by the two bodies . Similarly electric charges move from one point to another if there exist a difference in electric potential irrespective of concentration of charges at the two points . Thus electric potential determines the direction of flow of electric current . Charge flows from a body at a higher potential to body at a lower potential . By convention a positively charged body is always at a higher potential as compared to negatively charged body . If positive charges like protons or positive ions are free to move they would move from higher potential point to lower potential point. Usually the movement of charges implies movement of electrons. It is clear that electrons will move from lower potential to higher potential. This constitutes electronic current. Since the convention is that flow is always from higher to lower level, the conventional current is taken opposite to the electronic current. It should be remembered that conventional current implies that movement of positive charges whereas electronic current is movement of electrons.
NOTE : Conventional current was given much earlier when electron flow was known.And movement of electronic current is a modern concept.