Replacing existing ceiling fan, with on/off switch only to wall mount control of light/fan
To just answer your question, you need to attach your switch white wire to the bundle of white wire. You never, never want to separate the white bundled neutral wires and then energize the circuit unless you really know what you're doing... like as a professional.

Be sure to turn off your breaker before doing any of this wiring.

If you include pictures of the box and switches, we can help you further.