Discuss the advances that happened before the 19th century. The most important of these would be the theory of gravity and motion (as originally proposed by Newton in the 17th century), which was originally set up by Galileo's proof that all objects on Earth are affected equally by the same force.
Focus on the 19th century. Important advances in the 19th century include better understanding of electromagnetism, as well as the laws of thermodynamics. The electron was also discovered toward the end of the 19th century, which paved the way for understanding the atom.
Expound on the 20th century. The 20th century is when physics really came in on its own as a science. This includes the discovery of the proton and neutron (thus creating the modern day model of the atom), as well as Einstein's Special and General Theories of Relativity, which re-shaped how scientists approached space and time by showing that time is relative and is the fourth dimension.