Apple hardware connectivity
You may need to join the Apple MFi Program, however this does require you to be a registered company that can pass a credit check.

Who should apply to the MFi Program?
Developers who wish to develop electronic accessories for iPhone, iPad
or iPod using licensed components and/or software should join the MFi
Program. Companies, organizations, government entities and educational
institutions are eligible to apply.
Case developers, app developers
and developers of accessories that only use standard technology (e.g.,
Bluetooth Low Energy or standard Bluetooth profiles) do not need to
join the MFi Program.

If you just connect your device via Bluetooth then you do not need to join the MFI program.
All you will need to do in this instance is join the Apple Developer program for iOS devices, the cost for this is $99 a year.
To be able to develop the iOS app to communicate with your device, you will need an Apple Mac capable of running Xcode and costs will vary.