Very unpredictable. If aliens find us, then that would suggest that they would be more technologically advanced than us in at least some ways. Their morality might not resemble ours in any way, so it's difficult to say what would happen. They might try to exploit our planet, they might be disgusted by our behavior like in Space Odyssey, and they might just be interested in making friends . I wouldn't put it past humanity to try to wipe them out though, whether there is a good reason to or not.