Unfortunately, you can't work under a dietitian of your choice. You must be accepted in to an ADA accredited internship/supervised practice program. Each program is a little different, but it is quite competitive and somewhat challenging to find a program. There is a master list of the programs on eatright.org. Once you complete the program, you are then considered to be R.D. eligible and can work as a dietitian (under a limited permit) until you take and pass the exam to become R.D.
The other option after the 4 year degree is completed, I believe, you are allowed to take the DTR exam and work as a diet tech, until you find an internship in order to become RD.