first of all, take a plastic screwdriver and tap/hit it agains the side of the carb (hold the carb so it doesn't just twist) If it's just a bit sticky, or there's a chunk of gunk in there, this should dislodge it.
If that doesn't fix the problem, Pull the carb off the bike and take the float bowl off. Make sure the float pushes the needle all the way closed when the tab on top of the float that pushes the needle is just about level. gently bend it until it closes at level. Pull the needle out and blow some air through their to clean out any crud. If you don't know what I'm talking about, you'll need to get a manual.