I am an EE student, and I am working on my first quasi-freelance project. I am designing a PCB, and part of the design requirements is a square wave generator whose frequency is controlled by a Bluetooth receiver. I need a range of 20-1000 Hz. I know electronic circuit theory, but I have never worked with Bluetooth protocol before, and I don't know where to start.
Start with a commercial Bluetooth module that encapsulates the stack, read and understand all the application notes and datasheet. It will act more-or-less as a wireless serial port.
Implement the serial-port-controlled square wave generator with whatever technology is appropriate for the performance requirements- microcontroller with on-board peripherals, aux DDS or whatever.
If you are planning to sell this, read up on the license and other fees to use BT.
There are some Bluetooth modules which have widespread support by online communities. Eg. For HC-05 module you will find many readily working online projects/codes which will make your development easier. Other modules are mentioned Here .
You can also refer to the first chapter of this book for brief understanding of protocol.