One of the actions not mentioned by the previous post that helped lead to the rise of the US as a world power and its push towards imperialism was the further development of rich resources in areas that had previously not been available.  The increasing efficiency and access brought about by railroad construction, the influx of foreign immigrants as cheap labor and the wide open opportunity for advancement in terms of opening the west, etc., led to the economic boom domestically that helped fund imperialist ventures abroad.

Another action that helped lead to the rise of power was the passage of the McKinley Tariff that helped to further fuel the economic growth and a huge upswing in national confidence which helped to power the idea that the US should be taking colonies abroad and expanding its power base as well as helping to do things like "civilize the people who hadn't yet been" as the previous poster mentioned.

At the end of this century, the US moved towards being an imperial power.  There are usually said to be three motivations behind this.

1. A desire for military power.  The US wanted to have bases around the world so that it could project military force far from its own shores.
2. A desire for economic gain.  The US wanted colonies so that it could have captive markets and sources of raw materials -- places that no other countries could trade with.
3. The "White Man's Burden."  This was the desire to "civilize" the people in the places that the US took.