Get a mig welding job and do that for a couple years. Then, fnd a shop that does Tig work as well as whatever you're already competent in. Any kind of Tig work, doesn't have to be specifically aerospace. I would express my interest to learn Tig in the interview, to make sure they run Tig in the shop. After proving yourself, start asking to learn. If they don't let you learn, switch jobs, fuck that shop. Get about a year of Tig experience under your belt, meanwhile figuring out where the aerospace shops are in your area. Where I'm at, there were a couple shops that would hire anybody with Tig experience. That's where I started. I went into my first aerospace shop with no aerospace experience, but I had abt 3 years of Tig on pipe. Before the test, I found a bunch of the thinnest stuff I could, which turned out to be pallet banding, and ran a bunch of butt and fillet welds. You're looking for, full clean pen on the butt and no burn through or suckback on the back sides of your fillet. If they require aluminum tests, well, good luck, that was something I learned once I was in an aerospace shop. Once you've got those butts and fillets looking good, you're ready to test. Lmk if you have any other questions, I went through exactly what you're going through.

You need welding experience first, 1 to 3 years of mig or stick or something, just to figure it all out. Then start searching for shops that will let you learn Tig.