Have sex with them. Clearly that's why they're here.
Seriously, though, what moron came UP with this thing. First contact, if it happens, will happen a century before they actually GET HERE. Any aliens coming to earth have seen our broadcasts. They know as much about our analogue technology as we do. Digital tech will be more choppy, but they definitely got our analogue down pat. They know our languages. And SETI has shown them enough about our science and physiology that there's no doubt of our development on any cultural, biological, or technological level.
IF they show up. It's either because they want to strip mine our planet (doubt it- space is full of mud and metal- why fight a gravity well for supplies?)... they want us as slaves (why? They gotta have robots better than us)... or they want to find out if humans really are the biggest nymphos in the universe.
So, for the sake of our species' future reputation as the party capitol of the galaxy. That is what we must do.