What did the united states begin to do to their navy?

it began as soon as the English started to come over in 1607

Slavery existed in the original Thirteen Colonies before the creation of the United States in 1776. The first slaves were brought over from Africa after the founding of Virginia due to the slaves'...

They're all listed on this webpage: http://www.militaryfactory.com/ranks/index.asp But as I understand it, the basic gradations are: Seaman, Petty Officer, Lieutenant, Commander, Captain, Admiral

The abbreviation for the United States Navy is the obvious "USN."

Yes but you might be deranked or up ranked depending were you where in the Marines.