Becoming a doctor is a challenging prospect. It is not possible to do entirely online. You can take undergraduate classes or get an undergraduate degree online, but you cannot go to medical school online. There are a number of things you can do online, however.
Apply to a program that has an online degree--either an online university or an university that provides online classes (see the list of universities in resources)--in math, biology or chemistry. To apply, you will need your high school transcripts, SAT or ACT scores and a personal statement.
Try to attend a traditional school that has respected online classes, such as Drexel University or The University of Texas at Austin. Medical schools prefer transcripts from traditional colleges.
Excel in your math and science classes. Make sure to take biology, general chemistry, organic chemistry, physics, microbiology, biochemistry and anatomy. You will need to travel to a lab for most of these courses, either at your local college or at the online college.
Prepare for the MCAT, the medical school entrance exam. You can study and practice for this test online.
Don't ignore writing classes. There is a writing section on the MCAT, so make sure to learn how to write coherently and quickly.
Research medical schools. Visit the websites of the medical schools you are most interested in to find out if they have any specific suggestions for undergraduate courses they want you to take. Some common suggestions include foreign languages (especially Spanish), psychology and statistics.
Look for support and study groups that you can meet with off-line to help you with your most challenging courses.
If you are serious about becoming a doctor, you will be better off studying at a traditional university. Not only will medical schools be more likely to accept you, you will also get the nearby academic support you need.