If you step into your shower and you get a sulphur smell coming out of the drain it means that trapped Anaerobic bacteria is producing odor. Anaerobic bacteria thrive in areas where little amounts of oxygen is present (i.e deep in drains). Now what is causing the bacteria to occur is a separate issue. It may be that something is trapped there so if your drain is not working correctly then please call a Plumber.
This lesson is just how to remove the smell.
Ensure your drain is removing water efficiently first. If not you will need to clear it.
If your drain is working alright then tip down half a pint of my formula. Wear safety googles and gloves plus protective clothing.
Leave the chemical in the drain for 5 minutes then pour normal water down the drain to flush it through. The odor will be gone. If odor persists then either use an approved drain unblocker product or consult with a plumber.