Assuming that's directly copy-pasted:
ServerIP=sf::IPAddress(123, 456, 123, 456);// for example
Should probably be
ServerIP=sf::IPAddress(127, 0, 0, 1);// Connect to the local host
If you don't know what an IP Address is, it's basically your computer's location on the internet. "127.0.0.1" means "local host" or "this computer".