My bike has been sitting for a while, and I can't get it started
See here for more info on a bike that has been sitting.
This assumes the bike was running just fine before it sat, and that it sat for months. This also assumes that rodents haven't packed your exhaust pipes full of seeds or nesting. If the bike just sat for a few weeks, odds are that the carb bowls are just empty. In this case if you have a "prime" setting at the petcock (86-87 only), set it to prime and try again, otherwise keep trying to start with the choke at wherever it usually would be and throttle partway open until it fires. So that you don't overheat the starter motor, use the starter in short, controlled bursts of 5 seconds or so, with a 10 second "rest" in between.
1. Does the starter crank the engine? If yes, go to 2. If not, then check the battery and charge if needed. If the lights come on but the starter button just produces a "click" sound with no cranking, the battery needs to be charged. Keep the charger handy in case the battery wears down, or hook your battery up to a car battery (car off). If the battery is OK but the motor won't turn over, then there is either a starter problem (like it isn't getting power) or the motor is seized (and this would take years of sitting). Put the bike in gear and see if you can turn the motor by turning the rear wheel (grunt). If the wheel turns, then the motor is free and the starter is the likely problem.
2. The battery and starter are OK, so if you have starter fluid, try using it; if not, go to 3. Starter fluid is a quick and easy check if everything but the fuel/carbs are OK. Spray the fluid into the carb openings (read the can) downstream of the air filter, and try starting again. If it fires up, then you just need to drain the carb bowls and try fresh gas. See 4.
Note: Starter fluid is very flammable. Don't smoke, and use it sparingly, or your little Keihin carburetors could turn into flamethrowers.
3. Check for spark. Put the starter fluid can away; it is extremely flammable. Reach under the gas tank and pull one of the spark plug wires, with cap, out of the head. This is kind of a pain, but quick enough and thus worth it to do, especially if your bike sat in the damp. Plug a spare spark plug into the cap and hold the plug threads against bare metal on the bike. If you don't have a spare plug, put a metal screwdriver shaft down in the cap, and put some part of the metal shaft up close to bare metal, like 1/8" (3 mm) away. Grab the screwdriver with a glove or rag if you don't want a little shock yourself. Hit the starter button. If you don't see a spark, then there is some problem with the ignition, likely a short somewhere. If you do see spark, then the problem is definitely with the fuel or carbs. The spark might be hard to see in direct sunlight. Put the spark plug wire and cap back on the plug in the head and go to 4.
4. Drain and re-fill the carb bowls. Condensed water and dried fuel varnish can plug up the works enough to keep the bike from starting, although it often will at least sputter. There is an allen head screw under the carb that will let fuel drain out that little nipple under the carb (put a cup or some-such under there to catch the gas). If the gas in the tank is older than a year, drain the tank and put fresh gas in. Now put the petcock to "prime", if you have that setting, and try starting again. It will take some cranking of the engine for the gas to re-fill the carb bowls, so give it 10 tries. If it doesn't start, go to 5.
5. Clean the spark plugs, or put new ones in if you have them. It's such a pain to get at the plugs that you should definitely leave this until late in the operation, but still, it's easier than cleaning the carbs. If the bike still won't start, with good spark, starter fluid or fresh gas, and clean plugs, then pull the carbs and clean them.