What are the best paths to become a prosthetist if majoring in Bioengineering?

Hi Soledad! I think Bioengineering or Biomedical Engineering are great major choices if you was to be a prosthetist. I would make sure you have a good foundation in Mechanical Engineering as well, perhaps through a minor or electives. Another way you can prepare is to do a internship in a related field during your summers. I am not a prosthetist, but from a brief Google search, it looks like you will need a Masters in Prosthetics and Orthotics as well. Often with such a specialized field, you can demonstrate interest and experience by your extracurriculars, such as clubs, volunteering, etc. You might take up 3D printing as a hobby, or get involved in undergrad research in brain machine interfaces. 
In short, there's quite a bit of options for you. There isn't one path that is super common that I am aware of. Best of luck!