In order to connect to another computer, the destination computer must be configured properly. First, the computer must have a VNC server (and, optionally, an SSH server for secure connections) installed and configured, ready to accept incoming connections. Next, the network gateway device (router, modem) providing Internet access to the computer must be configured to forward incoming Screens connections to the appropriate computer.
The next few sections will walk you through the process of getting a computer ready for Screens based on which operating system it uses.
You may also be interested in using Screens Connect to handle some of the configuration work for you.