Swift is just a computer language. Your first job is to design what you want to do, regardless of the language. What platform(s) are your targets (Apple watches, iPhones, Android phones, Fitbits. etc.)?
What media are you going to use (Bluetooth, low energy WIFI, both)? What will your data stream look like? What are your data communication protocols look like (request/response, continual feed, etc.)? What does the UI going to look like on the device and the phone? Once you've started to get a handle on these questions, you can look at the infrastructures you want to support, the libraries and frameworks from the platforms you want to support that provide the low-level services, etc.. Then, you can start to code this all up in Swift, Kotlin, Java, Objective-C/C++, or whatever language suits your fancy depending on what platforms you want to support.
For Apple platforms, Swift is probably the best choice as the implementation language. For other platforms, there may be better choices based on what you want to accomplish.