The pole at the base of the handlebars goes into the front of the frame of the bike. At that point, where the pole goes into the frame, there is a screw or a nut that can be tightened with a wrench or an allan key.
Tighten the bolt that adjusts the clamp holding the handlebars. If the bolt won't tighten any further, get new a bolt and/or clamp. This is a safety issue.
to tighten handle bars, u need an ajustable wranch, then u need to adjust the handle bars so they feel good, then tighen the bolt holding them on the bike