Hi, If you want to put the hot tub on an existing deck then you will want to shore it up structurally. As the other respondents said, this is best done through and experienced professional deck builder. From my experience, I have found that it is often enough to put a triple 2x10 girder beam under the joists directly under the middle of the hot tub. This girder should have end and center posts with deep solid footings. The deck should also have plenty of diagonal bracing to prevent the whole thing from falling over to the side (wracking). IE: angle braces that go from the footing, up to the band joist. This is extremely important if you are adding weight like this to a deck that is more than just a couple feet off the ground.
Another solution which I had built for my parents was basically two decks. There was an upper deck that had about 4 feet of width all the way around the hot tub. There were built in bench seats on the outer 2 feet that were integrated into the railing. There was two feet wide to walk all the way around and a big hole in the middle that was just a tiny bit larger than the hot tub. The tub was sunken flush to the upper deck. There was another, smaller deck underneath that supported the hot tub. This deck was braced in every direction and was built about 4 times stronger than a regular deck (at least). No one has ever fallen into this pool arrangement. They use the heavy insulated cover when not in use (the same color as the stained wood deck) and they close a "fence gate" entrance to the deck when not in use that includes a do not trespass sign. All of this is important because in most cases all of the laws that apply to full sized pools also apply to hot tubs (ie: being safe and fenced off to animals and children when not in use). this arrangement is drop dead gorgeous. The most important feature is having a professional, licensed electrician install a gfci outlet for the pool's power supply. This stands for "ground fault circuit interrupter" and it is an outlet with the little red and black buttons. If you don't have one of these you can die from an electrical shock.