Hi Sonia,
The onboarding session should be designed as per the business need whilst few of the common practices ought to be incorporated.
History of the organization
An overview of the Business, revenue, market position, core Values, Mission
The Hierarchy, functional lines
Alongside, you may also think of having a session on soft skills based on your participants/audience. If the participants are freshers; don;t have too much of metrics oriented data on the first day of their experience. What can and will bring a fresher's involvement is ; a learning with fun. You may look at getting a session imparted on corpporate etiquette to make the employee aware of certain DOs and Don'ts, Certain business/social norms to be followed. The most important part as per my belief would be : Have activities, anecdotes, icebreakers, games etc.(Based on the audience).
Regards
Debashish