ok, as far as i can tell that's a threaded stem, so you need to remove the black cap on the top of the steerer tube / stem and that should reveal where you can use a metric allen key to tighten things.

if you watch this video it will show you how your headset works. you don't need to follow all those instructions, just the last steps (where the video should start, at 2m20s) to tighten everything.

so you need to buy a metric allen key of the right size. example