As of now, you need to complete an undergraduate DPD (Didactic Program in Dietetics) program and apply for/be matched with a dietetic internship. The internship is completed after you finish your required undergraduate coursework. The dietetic internship can be as short as 6 months or as long as 2 years. The longer ones usually include a Masters degree program worked into them.
In the foreseeable future (5ish years or so) a Masters degree will be required in order to become a dietitian.
Once you complete your dietetic internship, you sit for the RD exam. Once you pass this, you're a dietitian! Each state has their own licensing laws so you would need to look into that as well. For example, I work in Iowa so, in order for me to practice as a dietitian, I have to have my RD credentials as well as LD (licensed dietitian) for the state.
Hope this helps!