I want to contract a swimming pool in a residential area.

The normal way is: you need to have a company (with a business license, of course), and then sign a contract with the owner of the swimming pool. The contract clearly stipulates the contract term, cost, payment term, safety responsibility, decoration and equipment purchase, etc.

Then you organize people to start doing it! The upfront cost depends on the signing fee, the hardware level, whether to add necessary facilities, and what personnel you recruit and publicize, which others can't help you figure out!