An answer to your question depends a lot on how deep you're looking to learn on the topic.
Assembling a computer isn't very hard. If you boil it down to the simplist explanation, it's merely plugging things in.
Understanding how a computer works at a circuit level? That's definitely much more indepth than most of us go.
Google is your best friend really. You can't learn all this stuff ovenight, a couple days, or even in a week or two. It takes time. Plus, it's changing rapidly, it has for quite some time now and it doesn't show any signs of stopping soon.
As for sites, www.proprofs.com
has a good A+ prep program that will show you computer hardware and software basics. It'll talk about what the parts of a computer are, and basic ideas of how they work and their role in the system. As for software, you'll learn what an OS is, it's relationship to hardware and applications. Then it'll talk about Windows and basic troubleshooting.
If you're just wanting to know what video card beats another or what CPU is best, that can only be learned by asking questions, listening, and reading reviews of parts.