What should the protocol be for greeting extraterrestrials?

At the moment, if Aliens turn up, there is no governmental protocol when it comes to dealing with them. We have no way of discovering their intentions, of communicating in any way or even knowing how to behave around them. Do we surround a UFO with armed guards? It could be seen as weak not to or overly aggressive to do so. Do we send out a single diplomat, a whole team or wait for them to act first? If so, who do we send? And how do we greet them/ communicate with them/ introduce them to earth?

Someday we may need a set of guidelines for dealing with a first encounter, Reddit, this is your chance to write those guidelines.