Whenever I want to be exposed to the basics quickly, I usually start at course websites that have lecture notes and homeworks available for review. Maybe try something like <https://agora.cs.illinois.edu/display/cs231sp09/Schedule>? I must note though, that in general, I've found lectures slides without the lectures not too "good" in the sense of there being details left out.
In my experience though, I've found it best to learn my computer architecture from Hennessy and Patterson's books. To start, you may want to check out "Computer Organization and Design: The Hardware/Software Interface, Third Edition" from a library. For more detail about the broader problems in computer architecture, I recommend their book entitled "Computer Architecture, Fourth Edition: A Quantitative Approach". Google Books may allow you to preview some pages online.