The First Annual Hunger Games
There is no reference to the first Hunger Games ever in both the movie and the book series. It can be assumed that there were 24 tributes, a boy and a girl chosen from each of the 12 districts. As for mentors, it can also be assumed there were possibly none due to the fact that this was the first annual Hunger Games. It can also be assumed that this Hunger Games was the one to set precedent for the rest. As in, determining the amount of tributes, ideas behind mentoring, the reaping, etc.
Nothing is mentioned about this Hunger Games so fans are left to assume what could have happened.