Hello, I am an American and Polish citizen currently on vacation in
Poland. I have bought a car here and would like to bring it back with
me to the United States. I understand that I will need to have the car
converted to miles instead of kilometers. What else do I need to do to
legally bring the car back to America with me ? Your help is greatly
appreciated.

It very much depends on the make and model of the car you have bought.
 Meeting US specifications is not just a matter of having the
speedometer show miles and miles instead of kilometers driven, other
things such exhaust and crash testing for the model must be approved. 
Some car models that are not imported to the US by dealers have not
been crash tested (or weren't approved) and cannot be registered in
the States.

Here is a website that may be of help.  At the bottom of this page,
you can click to one that provides more information, and then to a
"vehicle eligibility list".

http://www.nhtsa.dot.gov/cars/rules/import/FAQ%20Site/pages/page6.html