Question

How do i register to start the biology course?

Answer 1

If you mean on OCW, you can't.
OCW gives you access to the course materials, and you can go ahead and access them directly, but there is no registration or "timed schedule" whatsoever (there may be suggestions as to how long it should take to learn X, but nothing forces you to follow them !).

There ARE online courses based on the material, though:

https://www.edx.org/course/mitx/mitx-7-00x-introduction-biology-secret-1768