Legolas: I can not go back.
Thranduil: Where will you go?
Legolas: I do not know.
Thranduil: Go to the North. There is a member of the Dunedain you should meet and stay with. His father was a good man, I have hope he will be a great one.
Legolas: What is his name?
Thranduil: He is known as Strider. His true name, you must discover for yourself.
Kili: I will not hide, when others fight our battles for us!
Thranduil: So this is the Halfling who ate my food, and stole keys from my guard.
Bilbo Baggins: Yes. Sorry about that.
Thorin Oakenshield: Farewell, Master Burglar. Go back to your books, your fireplace. Plant your trees, watch them grow. If more of us valued home above gold, it would be a merrier world.
Bilbo Baggins: One day I'll remember. Remember everything that happened: the good, the bad, those who survived... And those that did not.
Chosen answer: After the battle the Arkenstone was placed with Thorin in his tomb. It is shown that during the siege, prior to the battle, Thorin sends out a raven which returns shortly before the Dwarves arrive. This raven carried the message to ask for help. The mission to retake the mountain was not a secret among the Dwarves, Thorin had sought an army to help them from the beginning but only the few who went on the journey were willing.
Not quite, the mission to retake the mountain was organized by Gandalf and Thorin, who lived in exile with his family and small group of followers. Dain and other Dwarves did not know, as Gandalf never shared this information with anyone, not even Elrond. It was only after Thorin and Co. Actually arrived in Erebor that Thorin asked the ravens to travel to any Dwarf settlement nearby and gather the armies as the Elves and Men would no doubt come back to the Mountain. The raven was actually an old raven, so he only flew to the Iron Hills to get Dain.