What occupation does Dill decide he wants to pursue?
I think you are asking what Dill wants to be when he grows up. At one point he asserts that he would like to be a clown.
This matches his character for 2 reasons. On the one hand, he enjoys being an entertainer. The kids call him a pocket-merlin because he magically plays roles very quickly. On the other hand, he is a gifted and talented liar. He tells stretchers, big ones (according to Scout). And he wants to make people laugh.
Later in the story, he reports working on the railroad as an engineer with his dad (who we don't believe that he really has).