hi ya mate
is it an old school style stem with a shaft that goes down the head tube? or a new school style stem that clamps round the fork top?
an old style stem goes down the head tube and you tighten a bolt at the top of the stem (can be a nut or an allen key bolt) this tightens up a wedge at the bottom with wedges against the inside of the head tube and keeps it tight.
the new school type is a little diferent, you have to have the correct spacers underneath the stem to make sure the top of the foks is about 5mm underneath the stem, then you have a top cap that screws into the top of the forks and clamps the stem and headset down so it has no play, then tighten up the allen key bolts on the stem to clamp it to the forks