It might be better to use an Arduino with a spare HardwareSerial port for the Bluetooth module. Using SoftwareSerial on an Uno might slow the performance sufficiently to interfere with the stepper performance. And using the Uno's HardwareSerial for the Bluetooth module might make things too complicated for a beginner. A Mega would be my preferred option but a Leonardo also has a spare HardwareSerial port.
...R
Stepper Motor Basics
Simple Stepper Code
Serial Input Basics - simple reliable ways to receive data.