Watch how a football quarterback and a baseball pitcher throw a ball. Watch closely how their hand and wrist seems to turn very awkwardly, the opposite way that you would expect. Then go watch a top tennis pro and you will see the same thing. You are probably not doing this. This is called a wrist pronation.
 
The greatest reason why a pro can serve so hard! They are not throwing their shoulder joint and rotator cuff as hard. They are letting the ligaments in the wrist do most of the work. So, if you are struggling with your serve, go to your local tennis professional and let him or her explain this to you. You will be happy you did. You will hear things like pronation, continental grip, hit toss on the fall, etc. These are all things you need to understand.
 
Remember, everyone's serve looks and starts a little differently. That's normal. But putting the wrist snap in is probably 80% of what will allow you to hit your serve faster.
 
Good luck on the courts!