Listen or put out a “CQ ATV” call to make initial contact. A licence issued by your national authority is required to operate a television transmitting station, licence conditions will vary in different counties — in the UK this is administered by OFCOM.
To start communication, listen for a “CQ ATV” call or send one. You need a license issued by your national authority to run a television transmitter, and licensing rules may vary between different regions — specifically in the UK, these are managed by OFCOM.