Harriet Tubman overcame extraordinary adversity to become a historical figure, and so thesis statements about her early life would give a lot of material to work from. For example, she was put to work at age six, and from that moment on endured beatings and harsh conditions as she grew. You could write: "Because of her hard childhood, Harriet Tubman was well-prepared to overcome racism and brutality in her later years."
Another area is the unsympathetic view that other slaves had for her dreams; her first husband, John, was a free man but did not want to help Harriet become free. He claimed that his life was acceptable and there was no point inviting danger and the ire of slave-owners. You could write: "Despite the discouragement of those close to her, Harriet Tubman never stopped believing that she could be free."
Another area is the Underground Railroad, which was the route that Harriet took to escape slavery. She later became a "conductor" on the Railroad, helping other slaves escape; she claimed that over twenty missions, she never lost a "passenger" to surrender or betrayal. A good thesis for this topic might be: "Harriet Tubman's experience with slavery, and her knowledge of human nature gained through personal experience, made her a very effective ally to the Underground Railroad."