#1
I've often wanted to start building a very simple tube amp. What parts should I invest in? Anyone want to explain to me the general concept ?
Gear:
1980 Gibson SG Standard Cherry Red
Ibanez TM-71 Talman Artcore Semi-Hollow
2003 Tom Delonge Fender Strat Seafoam Green
Traynor YCV40
#2
Check some of these sites out

http://www.ax84.com/
http://www.aikenamps.com/

Do your reading first, and it would probably be a wise idea to work on smaller audio electronics projects (micro amps, pedals, etc) before building a tube amp, even if it is a simple single ended one. It is not a project for someone with 0 electronics experience due to the cost, potential for screw ups/noisy circuitry, and the fact that parts of the circuit have dangerous (can be fatal) amounts of current running through them.
#3
unless you know a lot about electronics I wouldn't attempt making one from scratch. I haven't made one myself I'm just saying a kit may be the way to go for the first one. most of the time when people are modding amps the swap out the OTs, so I'd say invest in a good OT and good tubes.
Current Gear:
ESP LTD MH-250
Ibanez rg470
Peavey ValveKing 112
Blackheart little giant head
2 homemade 112 cabinets
Boss SD-1
#4
^^+1

Do a lot of homeowrk before you attempt to build a valve amp.

Study simple schematics (Epi VJ), ask around on amp forums (sewatt, 18 watt, metroamp's forum, etc.).
Now it's 1984
Knock knock at your front door
It's the suede denim secret police
They have come for your un-cool niece