The easiest way to make a networked multiplayer environment is to use the Networking Starter component. The starting point is a single player environment. The only thing you need to take care of is that you set the Remote Avatar (see below).
To turn this in a multiplayer environment you need to add the Networking Starter component to the scene. You can find this in the Humanoid->Prefabs->Networking folder.
This script will take care of all networking. At launch, it scans the scene for Humanoids and recreates the Humanoid at each remote client, synchronizing all movements and actions over the network.
The exact parameters of the script is found at the page for the relevant networking solution: