Enroll and be accepted at an accredited university that offers a Bachelor of Science in Dietetics. This is typically a 4 year degree, but it can take longer depending upon your schedule.
Once you've declared a major of Dietetics, visit your adviser. I cannot stress this enough. They will help you to make the correct choices of classes. This will help ensure that you do not waste your time.
Get your degree. No matter what it takes, press on. Keep the end goal in sight. You WANT to become a Registered Dietitian.
Once you've graduated, you must take the national exam to become registered. After you pass the exam, depending on which state you reside, you'll need to get licensed. In most states, you must do these things in order to practice dietetics.
Complete continuing education to maintain your registration status. You must complete 75 contact hours of approved continuing education every 5 years in order to maintain your registration status. This is critically important. Failure to do so could result in loss of registration status. You'd have to take the exam again. Yikes!