All of the above suggestions are great. When I became interested in amps/electronics, I found this document http://www.ax84.com/static/p1x/p1-ex-theory.pdf
You'll need to know what filter caps are and how to properly discharge them before you start poking around inside a chassis. I'd start there then read as much as you can.
It will be helpful to have an amp you can work on - I suggest something with a simple circuit; I used a SF Champ. I took it apart several times for different reasons: changing filter caps, coupling caps... a bunch of mods, replacing tube sockets, etc. I studied up on what I wanted to do and then tried it out on the Champ. Somethings worked great, others required me to troubleshoot - a great learning experience itself. A
I think kits can be good learning experiences if you have done some homework beforehand. Otherwise, it can be "paint by numbers" and you'll put an amp together and still have no idea how or why it works.