Wifi Gateway
Could you be a bit more specific? Do you mean like a login portal that would ask for credentials in order to get access to the network (like in hotels)? If so, the easiest thing to do to secure your network would be to password protect your router with WPA encryption. If you actually want a splash screen that users must get by in order to gain access to the internet, you'll need to do some more advanced things. Could you please let me know what kind of router or server you are running? This is something that would need to be configured on the server side of things and not on a client Mac or PC.