I'm not sure what type of knife you want to make. Is it a live blade or trainer? If its a live blade aluminum won't work, as a others said, its to brittle, won't hold and edge, etc. If its a training blade alloy is better because it allows you to use thicker material for safety and to replicate the propper wieght, steel unsharpened trainers still have an edge because they are too thin.
Practicing making trainers will help in understanding knife design and dynamics, but the process is different. If you want to make a live blade I would suggest starting with light tool steel (lawnmower blade, old files, old machete, etc). I was never one for the "kit" approach. To me those are already done - all your really learning is how to put the handle on, you can get that from doing trainers. One of my favorite utility knives/bolos was one my dad and I made from an old lawn mower blade. It didn't have much shape to it, the only shaping we did was to taper one end smaller for the handle and round out the tip. We ground a very sharp chisel edge an wrapped leather for the handle. Wish i still had it. I guess what i'm trying to say, is don't start out trying to make a maserpiece, you'll be disappointed. Try and make functional tools (like my lawnmower bolo) that you can use, if you mess up, just keep grinding! As you gain experience, take the next step...forging, blowtorch, tempering, whatever.
Andy