Youtube mostly. I also discovered that one of the people at a local regular tech meetup teaches welding at a community college and he critiqued my welds and gave me some useful advice.

I've been using an AHP AlphaTIG 201xd, which seems to be a good machine for the price. It seems that the hard part initially is mostly just figuring out what settings to use to get a good weld. Beyond that, it's about getting used to how aluminum behaves, and figuring out how to position yourself and your work piece so you can keep your hands steady.

My welds aren't anything I would mistake for art, but they get the job done.