Hope this isn't too great of a necro bump, but its been a while since I've been on the forums, and I just saw your post.
I used to make armor for my medieval stuff many moons ago, and while I was never as good as Christian Fletcher, I could make some basic stuff.
I recommend starting in steel. Its heavy, and you may not want to use it for the final product, but its cheaper and easier to work than aluminum, and far easier to find replacement if you really screw up a piece. Its also fairly forgiving if you do make a mistake and have to work it back out. I also don't know much about annealing aluminum, but from what I recall you have to be a lot more careful with it than you do steel.
What you'll need:
Materials
Time
Patience
2 cross peen hammers, one unmodified, one you round the face off with a grinder to be your dishing hammer.
1 ball peen hammer
1 Planishing hammer
Recommended you have rubber or steel mallet on hand as well
Several sets of vice grips, or if you are feeling fancy, blacksmithing tongs.
Gloves.
Eye and Ear protection. Smacking on metal is LOUD, and over time can be just as bad on your ears as gunshots.
Either a swage block, a dishing stump, or a rounded metal surface to dish on.
A butane torch, or other heating device capable of heating steel to cherry red. This is for annealing the steel.
Some device capable of cutting the steel. Tin snips, metal shears, band saw, dremel attachments, whatever.
Grinder
Metal File
A guy I know uses an 8lb steel shot put ball welded onto a 1" steel bar stock to dish on. I used a dishing stump. I recommend a dishing stump or a swage block, as forming down into a cavity is a LOT easier to do than forming over a projection. Especially if you are free forming the piece.
Anyhow, that's more or less the basics.