I'd STRONGLY suggest you do a search on jetting, as was suggested, and educate yourself a bit before you start swapping brass.
The last thing you want to do is lean it out too far and have it seize on you.
Learn what jets cover what areas of throttle range and then figure out where you're bike is running poorly. Now you can start playing with brass.
Oh yeah, keep in mind that jetting is weather-dependent. So the jetting that works with today's temperature, barometric pressure, and relative humidity may not work with tomorrow's weather conditions.
Yes, there are ways to account for this stuff, but you need to walk before you can run.