To start with you can talk about the symbolism of and importance to the story of the convict in the marsh. Since this is the first action of the plot (plot moves forward through the conflicts that arise), it must be supposed that there is some overriding metaphor connecting the encounter with the meaning and theme(s) of the whole novel. Have you finished the novel? Or have you progressed only up to Chapter 3 in your reading? In either case, since the event is the first in the story, you would do well to point out its apparent importance to the overall meaning and theme(s).
If you have not completed reading Great Expectations, you can do no more than form a theory (also called a thesis) as to the symbolic meaning, importance of the plot conflict and overriding metaphoric connection to the meaning and theme(s) of the novel. Analyze what you can find that seems to bear upon the convicts presence or life (by way of illustration, new police officers in town, someone who was a victim of his crime, etc); is a contradiction to it (by way of illustration, perhaps an exceptionally good and generous person met in the bright light of a clean town or sunny meadow); people or events that are connected to him (by way of illustration, people who talk about him or seem to know about him, etc). Then construct what you think might be a reasonable guess at a theory of symbolic, plot and metaphoric importance to the story. If you have read the whole book, you'll be able to piece together and analyze what you know of the whole book.
From Chapter 2 you might talk about the psychological effects Mrs. Joe's manner of rearing her young brother is apt to have on Pip's ability to form, know and hold his own mind in the presence of strong influences. You might also talk about whether Pip shows inner strength when he denies himself food. Then, if you decide that Pip does have inner strength, you might talk about where you think it comes from.
From Chapter 3 you could talk about the moral significance of Pip's theft of food from his elder sister Mrs. Joe. What does this action reflect about Pip's character, what traits does he have? What does it say about Pip's sense of right and wrong? What does it say about Pip's ability to recognize the truth in a situation or in a person? What does it say about Pip's maturity and sophistication? Is he gullible and naive or is he wise and informed?