Let me clarify you that till now no government ever officially adopted a policy about a message from extra terrestrial intelligence (Aliens message). The United Nations Office for outer space affairs said that, their job does not include any issues regarding this extra terrestrial intelligence.
So with this we can say we are unprepared .Historically many people said that alien life exists. We might get a doubt how aliens will send us message. Of course there is a way if the aliens are technologically leading advanced life forms and capable of sending us a radio signal. Groups like SETI (the search for extraterrestrial intelligence) are actively listening. They have an ear (signal receivers) towards the sky and were waiting for the signals. SETI group are prepared. The SETI committee of the International Academy of Astronautics created a declaration of principles activities from extraterrestrial intelligence.
No government has officially adopted its recommendations, but if something happens it would be the first place authorities, This group (SETI) is the only handbook government have.
What happens if we receive a message from aliens?
The team which discovered the message will continue to forward it and alerts all the parties to SETI’s declaration. Next thing they are going to do is to confirm the message was real.
After the confirmation they will alert the Central Bureau for Astronomical Telegrams. Then the Central Bureau for Astronomical Telegrams will inform all over the world. First they start from expert observers. They would also inform UN secretary General (According to article 11 treaty).
The director of United Nations office for outer space affairs and the Chairmen of SETI post detection task force would be the ambassadors for Earth-Alien relations.
After receiving a message from aliens we need to send back a message representing Pi or Fibonacci sequence to develop relationship. Researches are going on and let’s see will they visit us or not.
Feel free to ask if you have any doubts. We respect your words. Have a nice day.