what happens at Bilbo's Hobbit hill?

Gandalf, the wizard, comes to visit, and Bilbo makes him tea. He transforms the tea into an organizational meeting for an adventure in which Bilbo is to play the central role as a professional thief. Of course, Bilbo is not interested in this and he has no experience, but Gandalf has brought twelve dwarves to the tea and the company disregard's Bilbo's protests. They also do a good job of eating all of the food in the hobbit's house.

I think this is where Bilbo has his call to adventure. Gandalf passes by his little hole and they have a small chat,

... I am looking for someone to share in an adventure that I am arranging, and it's very difficult to find anyone.'

'I should think so - in these parts! We are plain and quiet folk and have no use for adventures. Nasty disturbing uncomfortable things! Make you late for dinner! I can't think what anybody sees in them ..

I'm not sure what part of the story you're referring to; the beginning or the end, but the most significant changes to the hill are seen when Bilbo arrives home. Bilbo returns to find all of his belongings sold, a sword hanging over the fireplace, and a chainmail coat in the hallway (all symbolic of his own development). If you're referring to another section of the story please let me know.

I can see that everyone else was looking at the beginning of the adventure here....... and yes, the hill is where the adventure began!