Thanks for your answers. I did forgot to unscrew the stem... I managed to scrap one of the screw and I can't tighten the system enough to prevent it to move (I can't apply enough pressure on the stem). I went to LBS asking for a screw replacement but the two places I visited only carry the whole thing that is going for 30$!

This is the screw I am talking about:



The whole ''system'':




Crap...

Any chance I can find a replacement?
This new used bike is going to cost me a lot...