To answer this question, you may want to refer to a timeline. I have provided a link for one below, but there are also many others available online. You may want to look at each century and select four or five events in each. I am providing an example of this below.
During the period of 1619-1699, the Pilgrims arrived in Massachusetts, the Ch'ing Dynasty was established in China, the Taj Mahal was built, and there were Civil Wars in England, Scotland, and Ireland. During the 1700s, the First Great Awakening occurred in North America, the thirteen colonies declared independence from Britain, the Revolutionary War was fought and won by the Colonies, the United States established itself as a nation, and the Russo-Turkish War occurred. In the 1800s, the Industrial Revolution began, the American Civil War was fought, the Crimean War was fought, and electric lights were invented. During the 1900s, the Titanic sank, World War I and World War II were fought, the Spanish influenza pandemic killed millions of people around the world, and the world rapidly changed because of advancements in technology. In the 21st century, the United States was attacked on September 11th, 2001, the U.S. began the War on Terror, and Queen Elizabeth celebrated her Diamond Jubilee.
There is just a sampling of what happened during those years, and this is an example of how to identify important historical events. When selecting only twenty events, it will be up to you to decide what is most important to include.