Fuel, air and spark are always the basic steps to check when you're trying to figure out what's wrong. If you're missing one of them, the bike won't run.
Is there fuel? Is it clean, with no water in it? If there's good fuel, is it getting to the throttle bodies? Is the fuel filter clogged? Is fuel getting into the cylinders? You can check the flow out of the fuel pump to make sure the flow is correct; there are threads on here on how to accomplish that. If you pull a spark plug and it's wet with fuel, you know that fuel is getting all the way through the injectors into the cylinders.
Is the air filter clogged? Is there anything obstructing the flow of air into the engine? Is there a vacuum line disconnected somewhere, like the throttle bodies?
Is the battery producing enough voltage to start the bike? Is the gap correct for the spark plug? If you ground the plug against the cylinder head and crank the engine, do you see a spark jump across the spark plug gap? Check the condition of the spark plugs; there are charts on line that'll tell you stuff about what's going on with your bike based on what condition the plugs are in.
These kinds of checks will at least help you narrow down which subsystem of the bike isn't working. I'm not a gearhead, and there are lots of guys here way more knowledgeable than I am, but "fuel, air, spark" is always the place to start looking.
If you go to Youtube and type in "why won't my motorcycle start?", you'll get a nice selection of how to videos that should be helpful. Youtube has become my first stop when I'm looking for a possible solution to a mechanical mystery.
