The Question and Answer section for The Hobbit is a great resource to ask questions, find answers, and discuss the novel.
Thorin send Bilbo in to investigate first because Bilbo is the group's official burglar.
The Master of this region is familiar with the prophecy that foretells the reclamation of Smaug's horde of stolen treasure. Accordingly, Thorin is heralded and celebrated as a hero, for he is the descendant of Thror, King under the Mountain.
Bilbo felt that stealing the Arkenstone made him a true burglar...... he felt that keeping it was justified as it was promised that he could pick and choose his own share of the treasure. He hid the stone because he knew he was doung wrong.