If you need to understand computers, as in building them (as your post makes it sound) I would suggest starting with desktops.
Notebooks are a lot trickier, and a lot less flexible than desktops. They are mostly Mainboard and LCD panel, which are not easy to come by other than through the manufacturer (and expensive!), at least not if you want them in working shape.
You can swap out HD's and RAM easily enough, but the rest is fairly specialized. that is why technicians get specific certifications, for example Toshiba certified, etc...