If you can program in C at the level of K&R, your first step is to work through Computer Systems: A Programmer's Perspective [0].

Coursera has a course called the Hardware Software Interface [1] which covers the same material if you want lectures and forums and the other benefits of a MOOC.

[0] http://csapp.cs.cmu.edu/

[1] https://www.coursera.org/course/hwswinterface