The carburetor controls the fuel-air mixture that goes into the combustion chamber. Too much fuel = too rich, engine will run like crap, especially when cold, and the spark plugs turn black and moist from the unburned fuel. You need to lean it out, lower the amount of fuel the carb is letting in. The process is pretty simple once you know where the mixture screw is. To find that you're going to have to do some searching on google or something. Your best bet is to find a service manual for your bike and go from there.
Or you could just take it to a competent mechanic and ask them to point it out to you.