just to explain quickly.
jets are little nozzles that sit inside the carb, they deliver a set amount of fuel into the passing airstream going through the carb, creating a fine gasoline mist, the mist must consist of a certain ratio of fuel to air for the engine to deliver maximum performance AND reliabilety, to much fuel (rich) and you will lose power 'coz the engine cannot burn it all, to little fuel (lean) and you will also lose power, and you can get detonation (pinging) and in worst case scenarios destroy your engine.
the jets on most carbs are called : the main-jet, the pilot-jet, the needle-jet and the airscrew. and most carb-manufacturers use theire own jets, but all decent shops should have jets in stock, just take your carb to a shop and they will help you.
and how to jet is a complex thing to explain to someone. jetting depends of temperature, altitude, humidity, setup of your bike and so forth, it's a process of trial and error really, you try on jet and see how it feels, then you try another one and see if you notice a difference.
do a search on google and you will find heaps on guides to jetting.
