If you are buying Forever 21 shirts and printing on them for your own personal use, you are ok. Once you have purchased it, you are free to do what you want with it.
But once you start reselling them, it gets a little tricky. The fact that you are decorating and reselling their branded product, they are definitely going to care to some degree. Whether they choose to take legal action is up to them. You should definitely use some discretion and perhaps consult an attorney, because the reality is, even if what you're doing is completely legal they could still bring action against you.
Another thing to consider is pricing. If you buy shirts at retail prices, then decorate them and mark them up to resell, your retail price might be pretty high. If you do intend to start selling shirts, you may want to start looking into buying blanks at wholesale prices. They will be cheaper and won't carry the legal risks.