Question

when do I start

Answer 1

The course is a self-paced course. There are version every year in edx.org, which is a special version that gives you a free certificate if you pass it.

The edx version already started one month ago, but if you want to learn, not the certificate, you can do it and take your time.
https://www.edx.org/course/mitx/mitx-6-00-1x-introduction-computer-2841

There is also the self-paced way, which can be done in the next link.
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00sc-introduction-to-computer-science-and-programming-spring-2011/