Salesforce provides a great Lightning Transition Assistant to help migrate your org to Lighting as an optional interface.  You can get to this tool through Setup.

To use the Lightning Experience interface, a user will need to have the Lightning Experience User permission enabled.  This will allow the user to switch between Lightning and Classic.  Without this permission, the user will just see Classic.  The best way to do this is under a Permission Set where you set the Lightning Experience User permission, and then add users to that Permission Set as you roll out Lightning to different Profiles.

There's a great help article for how to do this at   https://help.salesforce.com/articleView?id=lex_enable_users.htm