First you need to know what options your particular installation will allow. Your contractor may be restricted by the existing structure of your shower.
Ideally, the steam head would be located in one of two locations for the environment you describe: either on the back wall, roughly 12" from the right wall and 8-12" off the floor; or centered on the right wall itself, roughly 8-12" off the floor. The control should be located near the seat and roughly head-height or 4' off the floor.
Read the following information for more detailed information on steam heads http://www.steamsaunabath.com//catalog/all_about_steam_rooms_head.php#head