Depends on the quality you want to do. First and foremost is sourcing the blanks. Gildan is big. Keep in mind that the brand name will be on the tag, you'll want to do something about that if you actually want to make fakes. As for printing, you can source it out to a print shop, or you can set up with a 4 color silk screen machine, they're a lot cheaper now than they were once, but you need space. Of course, you'll still need the blank screens, which are their own hassle. Sports team stuff will be easiest to unload, and carry the highest profit margin. If you can get ahold of some fake hologram stickers of authenticity (and fucked if I know where to get those), that really seals the deal.
Fact: people can tell iron on shit from a glance.
I actually sell tshirts for a living, so I'll tell you this, unless you have a sales plan, it a phenomenal pain in the ass to try to turn inventory into $. I've got about 12,000 shirts just in my floating inventory, and you've got to get them printed in batches of 72, minimum, to keep the printer price down. It's usually 72 per box, for each size, anyway. Hope you have storage space.
No shit, I'm thinking of buying a cheap house in Detroit just to keep my storage costs down.