Have you tried taking it back with a reciept for a refund?
I don't know what their policy is, but they might give you your money back or exchange it for another shirt.
I don't really know what you were doing in the shirt which might've stressed it to tear, but that's not a very good quality shirt if it gets a hole in it following very little wear.
You also may want to check that it's the right size for you, and it wasn't just slightly too small and you caused it to rip.