Start constructing an UDP datagrams to a remote host specified by IP address and port. Call write() subsequently to construct the datagram content. Then call endPacket() to actually send it.
udp.begin(1234); // start UDP on port 1234
udp.beginPacket(IPAddress(127, 0, 0, 1), 56); // start a packet sending to IP 127.0.0.1 on port 56.
udp.write(0); // write a byte of value 0 to the packet
udp.endPacket(); // actually send the packet.