I would explain humanity by first trying to make sure they take notice of the signal, are able to decode it, and then follow it with instructions on constructing a very basic medium of communication and how to use it to reach us in order to signal, beyond a doubt, that they've received it, and the extent to which they were able to decode it, which would also act as a test of their intelligence and overall development.
If that was successful, I'd make sure as many people on earth as possible could have access to this communication channel so that no one person could hold a monopoly of what is shared or not shared, and also so the Aliens can make up their own mind.
I'm not one to be afraid, since this Earth could at any point have an extinction-level event, and whatever information could come out of that contact may well be necessary to ensure some chance of survival and individual/societal/technological development beyond what's currently known.