How did the author get the message across to the reader in A Tale of Two Cities?
Your question is not very specific - what message might you be talking about?
The setting for this novel by Charles Dickens was during the time of the very bloody French Revolution which began in 1798 with the storming of the Bastille. As with other novels by the author, the chapters came out one by one, in a literary magazine. That is why each chapter can stand alone as a kind of short story in itself. There is a lot of action and characterization and the interesting plot draws you in.
You need to look at the analysis here on eNotes to decide upon which theme you want to explore. There are three main contrasting themes discussed: order/disorder; death/resurrection; and memory and reminiscence. Once you decide on which of these you want to write about, then you will have to figure out the ways that Dickens got his message across. Look in the novel for examples of plot, suspense, characterization, figurative language, etc. - those are literary devices that authors use to get their messages across.
If you can be more specific, perhaps we can help you more.