building a pc from scratch is easy if you spend a little time learning about it. there are various youtube videos on the "how-to" and people in our "hardware>systems" subsection can recommend builds.
i've seen a 13 year old asking questions on how to build one: if they can, you can. just remember to take your time, do your research and if you have questions ask them. never assume anything.
taking apart an old computer and putting it back together might familiarize you with the internals and basic layouts but it is not required. if you happen to have an old junker laying around then you might want to have a look inside but i wouldnt go out and look for one.
youtube videos, forums and google search is more than enough for the average person to figure it out.
also do a forum search... we have answered this question in detail so many times already. i know i've listed a complete checklist on one of the posts.