When someone starts a new job, it is a difficult time for everyone. A new person joins the team and impacts the dynamics, the new individual has to deal with all kinds of change and unknowns, and the organization has to onboard the individual to get them up to speed quickly and build the relationship so the employee wants to stay.
The most important thing that a manager can do for a new employee is clearly define the expectations. Everything from start and stop time, to how to communicate, to company rules, and everything in between.
Too often, managers don't do a good job of this. Mostly because many of the things they need to communicate are the things they do naturally, so they don't consciously think of them. But everything is new to the employee - down to where the bathrooms are and where to get coffee. Considering everything is an unknown, the more a manager does to eliminate those unknowns, the better.
How can you do this?
Make a list of all the things you think a new employee needs to know. Focus on the smallest and most mundane things - where to park, where to hang up your coat, do most people go out for lunch or eat in the cafe (or at their desks), etc.
Ask other employees to do the same - particularly ones who may have started recently. Ask what things they wish they would have known when the started. You'll be amazed at the things people remember not knowing.
Communicate to the new employee that they can ask you or anyone in the department anything. There will always be things that come up and you want them to feel free to ask.
The better you do at setting expectations prior to and on day 1, the more successful the new employee will be.