Rotation of a object depending on where the joystick is facing

This is a very broad question, and as such I can only really give a broad answer. We can figure out the direction the joystick is facing by taking both horizontal and vertical axes and combining them into a 2D vector. From there, we can use the LookRotation function of the Quaternion class to figure out a rotation that points in the direction of that vector and use that to rotate the target object;

 //Assuming you have setup the Horizontal and Vertical axes as joystick axes in the input settings.
 //Also, must be a Vector3 because Unity is at its heart a 3D engine and all rotations are in 3D, so just leave the z value at 0
 Vector3 dir = new Vector3 (Input.GetAxis ("Horizontal"), Input.GetAxis ("Vertical"), 0f);
 
 //Make sure we actually have a direction, and the joystick isn't stationary
 if (dir.magnitude > 0.05f)
 {
     //Find a rotation pointing in the direction of the joystick and apply it to our transform
     transform.rotation = Quaternion.LookRotation (dir);
 }