We come in peace, yo

If aliens announced their presence on earth tomorrow

First up
Assuming they're friendly, we'd have a heap of questions.
How did you get here?
Where did you come from?
How long have you been around the galaxy?
How did you find us?
Have you been here before?
What is your planet like?
Are there more alien races out there?
(and of course)
Are there any hot alien chicks? (cos Captain Kirk wants to know)Secondly
How would the world respond?
Would we put aside our differences on Earth to form a world alliance to explore the stars?
Somehow I doubt it. Cos who freakin' cares about some alien planet 10million light years away, when your neighbour has been trying to blow the crap out of your for 50 years.
It would cause some massive debates in science, religion and among policy makers.
You'd have to think the USA, with it's strong commitment to a space program, would be ahead of the game. But then France and Germany have spent some money in this area, while Japan and China have been spending cash too. India has also announced a plan to put a man on the moon. So there is some expertise around.

Thirdly
What could we learn from the aliens?
New power sources? Environmental control? Possible medicinal advances? Obviously new technological information (they did reach our planet, after all).
What dangers are out there? What should we avoid?

Fourthly
The future - where do we go?
Are there new worlds to colonise? Humans have always been explorers, after all.
Is there trade and commerce to be had, money to be made?
So yes, I've been watching a fair bit of Stargate Atlantis ;)