I assume you want an oUF layout, so I'll answer your questions based on that in a mixed order.
If you want an oUF layout you'd better pick on that looks as close as possible to want you imagine. You have to get oUF separately unless it is included in the layout you want.
oUF_Classic is by haste, the author of oUF, and is a basic reference to how to make oUF layouts. A lot of people on this forum use oUF_Simple by Zork as he has made a nice tutorial for it with commit history and comments on google code (which itself is the definition of fugly, come on Zork, move to github
). So it's up to you with what you start as a learning reference, choose what you understand better.
oUF plugins are addons that extend oUF's capabilities. Those are like third party oUF elements and you have to add support for them in your layout. This is in most cases trivial to do, but you'd better care about that later.
As for the last question, it's always better to have tried to do something yourself. Than post your code, any lua errors it may produce and an explanation why what's happened is not what you wanted.