If we made first contact with extra-terrestrials and developed interstellar space travel...

[Cowabungaa]

Are they hostile? Arm up and get ready to kick some ass.

Are they friendly? Establish diplomatic relationships, trading routes and most importantly; scientific cooperation. If you want to learn about a certain planet, you shouldn't ignore the knowledge of an advanced species which has lived on that planet for thousands or even millions of years.

Where do you draw the line between species that have sapience and species that do not?

That's horribly easy, actually: put them in front of a mirror. [http://en.wikipedia.org/wiki/Mirror_test] Of course it's a rather crude test, but the basics are sound: if you recognise yourself as an individual, you're self-aware.

If they are far weaker then us:
Militaristic expansion!

Humanity shall expand across the entire Galaxy!

(I'm completely serious here)

You keep surprising me Teddy. No wait you don't, but at least you keep confusing me. It's way easier to just work together with other species (provided that they want to), and it's also a lot safer for us and them. I don't know about you, but I rather not send our boys to battle when they don't need to.

Now if diplomacy fails and we do need to...FOR THE EMPEROR!!! Which means that we also need to elect a God-Emperor first. Any suggestions?

 

[Brackev]

I'd be joining them. We humans have already tapped this rock out and we need to expand. The Aliens would naturally already claim all the other planets in this solar system. What's left for us here but old memories. Join the aliens in their own game and take over other solar systems while there is still a chance of profit.

 

[RicoADF]

*SNIP*
what exactly relations would you try to make with aliens? and what lifestyle would you seek out with the new galactic society you'd be living in? *SNIP*

I'd want to be friendly with everyone possible, but reality is that humans would end up fighting everyone, and we would either crush the other races or be crushed by them.
Lets be honest here, the general public ain't the friendliest bunch.