If everything went smoothly up to now, you should be able to start up OpenVPN and connect.
On the Server
Go to OpenVPN GUI in the system tray and then click connect. It should now successfully connect and display that it has an IP address.
On the Clients
Once the server has been connected, you should be able to connect the clients. They should be able to connect to the VPN even when on the same local network, but testing from a separate network, like a neighbor’s wifi (that you have "permission" to use, of course), is preferable.