First I would start by researching the endless threads on here that discuss that very problem. This is a problem that has been hashed to death.
Then begin by making sure things such as your intake and carb are properly tightened. Look for oil seeps coming from all the assorted gaskets on the engine such as the crank case and such. Then start the bike (with the choke off) and spray something like wd40 around the joints that would be likely to allow air in. When you hear a change in the motor you've likely found the culprit. Be methodical, take your time, and you'll get it straightened out eventually.