What were some policies that reflected America’s imperial ambitions?
While the United States had spent much of the nineteenth century following Washington's Farewell Address advice of staying out of foreign entanglements, by the end of the nineteenth and beginning of the twentieth century, the US had become a wealthy and powerful country and was poised to become the next superpower. As a result, the US began to flex its muscle on the world stage. It was particularly interested in establishing a sphere of influence in Central America and the Pacific. As described in the other answer, it acted aggressively to gain control of places like the Philippines and exerted control over Cuba. It also annexed Puerto Rico.
Although the US had stayed out of Central American affairs in the nineteenth century (except for Mexico and Cuba), it increasingly involved itself in the region as time went on. For example, since it wanted to build the Panama Canal, it backed a 1903 rebellion that led to "independence" for Panama. The US then proceeded to control the country and set up the Panama Canal Zone as a wholly American owned district so it could run the Panama canal, which the US built. The US also interfered with sovereignty in Haiti and Nicaragua.
Haiti and Nicaragua were part of the so-called "banana republics" that the U.S. controlled for the sake of U.S. business interests, such as the United Fruit Corporation, which had a large investment in banana, sugar and other crops in Latin America. While the U.S. did not actually make these countries colonies, it used force to make sure they aligned their policies with U.S. interests. To this day the term "Banana Republic" indicates a puppet state essentially owned by corporate entities.
There were many American policies around the turn of the 20th century that showed that the US had imperial ambitions. America was, at that time, taking direct control of some areas of the world and exercising indirect control over others. Some examples include:
The Spanish-American War. This war was fought in part to show that the US was a major power. As a result of the war, the US took control of Puerto Rico, Guam, and the Philippines. It took indirect control of Cuba as well.
The war in the Philippines. After getting nominal control of the Philippines from Spain, the US fought a bloody war against Filipinos who wanted independence. The Filipinos had thought that the US defeat of Spain would mean independence for them. When they found it did not, they fought the US and the US crushed them and took control of the Philippines.
Annexation of Hawaii. The US tacitly backed the overthrow of the Hawaiian monarchy by American planters. Later, they took control of the islands outright.
The Open Door Policy in China. Here, the US demanded that it be accorded the same powers and rights in China that other imperial countries had.
These are some, but not all, of the policies that show that the US had imperial ambitions at this time in history.