Everyone here is presuming that aliens will come to either "farm" us and our planet or to say "hi" and share their technology with us, personally I doubt it would be for either reason
If they haven't visited us already, and there is some evidence towards the fact they have. It is much more likely they will be here to observe us and to judge our nature, and that does not bode well for us...
Look at us as a species, while there is such a thing as human kindness, a loving side to our nature, it is far overwhelmed by our aggressive side, do you really think a superior race (and they would have to be superior to get here) would want a species like us travelling the universe? Look at how we treat ourselves, we can't even get along with our own kind, we can't even get along with the other animals on our planet, do you think they would want us getting technologically advanced enough to travel from star system to star system?
If they come here, it will be to stop us getting out there...
To paraphrase the film aliens, if they examine us they will just think "Take off and nuke form orbit, its the only way to be sure" We need to learn to live with ourselves before we encounter aliens, we are far to dangerous and aggressive for them to allow us out there