Let's look at this logically.
If we are visited by alien lifeforms, chances are they will have come for our resources on a planetary scale. That, generally, is not too good for us humans. There are three main thoughts on this scenario:
1. We fight them and win. Highly likely, with their advanced technology, this isn't going to happen realistically. Chances are, all we will do is scorch the earth with nukes in an attempt to take them out, and screw things up for ourselves in the long run.
2. We will be exterminated. This may happen directly (i.e. an all-out strike) or indirectly (i.e. by taking our atmosphere or poisoning it by some means).
3. We are captured and put to work harvesting our own planet for them. We, in effect, become their slaves.