Unless you have a very special joystick and a very special bluetooth module you can't just connect a joystick to the TX pin of a bluetooth module.
Instead you have to have the joystick (I am assuming a 2D analog joystick) to a pair of analog inputs on (for instance) an Ardiuno, which then samples the analog values. It is then up to you to decide what you do with that sampled data - in your case you want to send it out via serial to the bluetooth module (I assume something like a HC-05 or HC-06). What that data is, and how you interpret it, is up to you.
You could send the data as plain text, such as:
Each line specifies a pair of analog values and is terminated by a line-feed. The receiving end then needs to read each line, split it on the
,, and interpret the two values as X/Y readings.
That's quite inefficient, though, and there are better "binary" methods that use less bytes to send the readings, but that is more advanced.