Oh this is good stuff!!
Just repeating what everyone said:
Get flush muffs or a bucket of water before you ever attempt to start the motor. The muffs hook up to a standard garden hose. You put the muffs on the water intake of the lower unit. Be sure to fully cover the intake ports. Before starting the motor and turn the hose on full blast. This will help ensure the motor gets water circulating. If you crank the motor or run the motor without water 2 bad things will happen. First the impeller (rubber water pump) will burn up and may break apart. And two, the motor will very quickly overheat and self destruct.
If you use a bucket of water (most likely a large trash bin) you will need to make sure the motor is covered up a few inches above the anti-ventilation plate, thats the horizontal fairings on the lower unit. You might also want to make sure that you constantly have water in the bucket as the water level will go down as your running the motor.
After you've started the motor you need to see a pee stream of water coming out of the top part. This is a water flow indicator. If it isn't flowing it could mean 1 or possibly 2 things. First it could mean that the motor isn't getting the water flowing through to cool it. If this is the case immediately shut it down. Or the 2nd could mean that the pee stream indicator could be plugged but the motor is possibly getting the water coolant. The pee indicator in most cases is just that, an indicator. Its not necessarily the definitive that the motor is or isn't getting the right flow of water. One of the definitive indicators that a motor is overheating is if the head gets too hot to touch. If you can put a finger or palm on the head of the motor and its more than warm but not too hot to touch you're probably ok. If it gets hot like boiling water off hot, its definitely overheating if not already overheated.
When I get a new motor the first things I inspect are the impeller, thermostat, visual inspection of pistons through spark hole, compression, and leak down test on each piston. You should definitely replace the impeller first thing before even attempting to start the motor. Impellers are supposed to be replaced every year or at most every 2 (at least thats what I've been told). Its a high wear item.
Well if you really want to understand your motor or DIY, the best thing is to get a repair manual and flip through it.
Just some of my $0.02