make sure the tank is empty before you do anything. (i assume since your valve is open that it's open your tank is also empty)
now look at the piece on the very end of your tank where the threads are. you should see two allen screws on the side. loosen them. they'll be hard to get off because they're lock-tited on. Then get a strap wrench or a vice and twist the end off. from there you'll run into all sorts of fun stuff. make sure to make not about how it came apart because you'll be sol if you don't.