The overriding idea in any piece of fiction is known as its theme. It tells us what the piece of literature is about, what happens, what ideas the writer wanted to convey, etc.
I can't help you with your exact thesis, as I do not know your preferences for it, but your thesis would be what the main point of the paragraph or what it is about. Make sure both your first sentence is stating your thesis and the first sentence of your last paragraph is restating your thesis (if it is only 1 paragraph, restate your thesis at the end).
Writing grand tales about Tom Sawyer, Huckleberry Finn and the mighty Mississippi River, Mark Twain explored the American soul with wit, buoyancy, and a sharp eye for truth. He became nothing less than a national treasure.
Samuel Langhorne Clemens, better known by his pen name, Mark Twain, was born on November 30, 1835, in the tiny village of Florida, Missouri, the sixth child of John and Jane Clemens. When he was 4 years old, his family moved to nearby Hannibal, a bustling town of 1,000 people.
John Clemens worked as a storekeeper, lawyer, judge and land speculator, dreaming of wealth but never achieving it, sometimes finding it hard to feed his family. He was an unsmiling fellow; according to one legend, young Sam never saw him laugh. His mother, by contrast, was a fun-loving, tenderhearted homemaker who whiled away many a winter's night for her family by telling stories. She became head of the household in 1847 when John died unexpectedly. The Clemens family "now became almost destitute," wrote biographer Everett Emerson, and was forced into years of economic struggle—a fact that would shape the career of Mark Twain.
Sam Clemens lived in Hannibal from age 4 to age 17. The town, situated on the Mississippi River, was in many ways a splendid place to grow up. Steamboats arrived there three times a day, tooting their whistles; circuses, minstrel shows and revivalists paid visits; a decent library was available; and tradesmen such as blacksmiths and tanners practiced their entertaining crafts for all to see. However, violence was commonplace, and young Sam witnessed much death: When he was 9 years old, he saw a local man murder a cattle rancher, and at 10 he watched a slave die after a white overseer struck him with a piece of iron.
Hope this helps:)
