NASA astronauts must be US citizens: see this document. In the past this resulted in some individuals changing citizenship to qualify. However, the international collaborative nature of spaceflight projects since Apollo has meant that many foreign nationals were trained by NASA and flown on NASA vehicles.
In the early days of the Shuttle program, nations that launched comsats from the Shuttle could fly a crewmember as a Payload Specialist...Saudia Arabia and Mexican nationals flew under this program.
There was a entire Shuttle Spacelab mission funded by West Germany; three West German nationals flew as Payload Specialists on this flight.
The deep involvement of Canada in the Shuttle and Station programs, through that nation's extensive contributions in robotics, lead to several Canadian nationals flying multiple missions.
In the 90s many European and Japanese individuals were trained by NASA to fly as mission specialists, especially in Groups 13 and 14. This was all handled by intergovernmental arrangements and the individuals were astronauts of their respective space agencies, not NASA.
Here is a complete list of international astronauts, defined by NASA as "those individuals from international space agencies who have trained or served with NASA Astronauts."
That's the history. In pursuit of your dream, you should know that unfortunately the number of astronauts needed by NASA is going to be very small for a long time, due to the long duration of missions flown to the ISS and the long delay until any other kind of NASA human spaceflight missions begin. But the bright spot is the possible growth in commercial spaceflight in the near future. Keep yourself in top physical condition, study medicine, engineering, or science and excel at it, obtain a pilot's licence if possible, and follow your dream! I salute your ambition!
