Total US population at ten-year intervals (when the formal census is taken).
Charts which show the population in total and broken down into broad demographics. Notice that population in the West is listed from 1850 onward -- because that was the year that California became a state.
This is filled with links to more information. Be sure to read the section called Conclusions.
Here is an online book regarding the population of the US. If you decide to read the parts of the book that you cannot find in this preview, be sure to go to your local or college library and find the real thing.
This is reference to a book that promises to give you a really good look at the US just as the new nation's Constitution, etc., were being hammered out.
These sources will help you partially. Since you are in an AP class, however, YOU need to learn to conduct searches and research!
If you need help learning how to search, including how to choose good search terms, go here and scroll down to the links under HOW TO SEARCH THE INTERNET: