Reinforce the foundation of the floor where the hot tub will be located. If you are building a slab for the hot tub, it should be made of concrete at least four inches thick and reinforced with steel bars throughout. The area should be able to withstand at least 90 pounds per square foot. Hire a contractor to confirm than any existing flooring will withstand this load.
Install sufficient ventilation. The room where the hot tub will be should have continual ventilation to help move moisture out of the room and prevent mold or mildew. You may also want to install a dehumidifier to remove additional moisture from the air.
Consider and plan for drainage of the hot tub. Some hot tubs have a hose connection that allows you to run a garden hose from the tub into the yard or other drain to remove the water from the tub. Or you can install a drain in the foundation if you are building it. This is the most expensive option. An alternative is to purchase a submersible pump that will pump out the water when needed. However, you will need to remove the last inch or so of water with a wet/dry shop vac or by hand.
Have an electrician run a dedicated 240 volt circuit to the room where the hot tub will be. Be sure that the outlet is grounded and that there are no exposed wires anywhere in the room.
Install the hot tub according to the manufacturer's instructions or have the manufacturer or other licensed spa specialist install it for you.