What would happen if we discovered an alien civilization?

I`m sure that I am not the only person who has ever looked up at the stars at night, thinking to myself ``someone is out there, looking up at me as I look up at them``. Now, considering the size of the Milky way galaxy alone, there must be somewhere out in space other than Earth where life has developped. Now, hypothetically, let`s say that we were to run into said an advanced alien civilization. What would be the implications? How would a cultural exchange work between our two races when there are numerous cultures on Earth, and possibly on their planet. What about exopolitics and economic? Would peace be an option, or would a war be inevitable? There are endless possibilities here people, so voice your opinions please.