The problem with this is that many players, especially beginners and intermediates, become too conservative with their 2nd serves. They put the bare minimum amount of power on it and then are faced with a powerful return from their opponent.
Ideally, you would get your first serve in every time, although even professionals don’t have near 100% accuracy with their first serves. However, there are a number of ways to improve your accuracy.
Keep the Same Serve
To continue on the topic of consistency, you should stick to the same technique for all your first serves. Instead of using a variation of first serves that are flat, have top-spin etc. learn one. Master that serve and then in the future you may choose to implement different techniques, but make sure not to get ahead of yourself.
Use the Proper Grip
Practice and Preparation
A single serve isn’t going to leave anyone gasping for breath. Though if you do dozens of serves back to back you’re definitely going to get your heart pumping.
Serving a tennis ball is requires an exertion of effort because you are using a number of different muscles in your body, and are generating a ton of power. When done continuously it requires you to be in proper shape.
The last thing you want is fatigue to bring down the quality of your serve in a match. Not only should you be doing exercises to enhance your endurance, but you should also practice serving large amounts of balls.
Set Up Targets
A surefire way to increase accuracy is to set up targets in the service boxes across the net. Set up 3-4 different targets in each box, varying them towards the center, corners, service line and singles line. Choose targets that can easily be knocked over like pylons or a pyramid of tennis balls—even a water bottle will do.
When serving, serve much softer than you would do in a match. The point of this exercise isn’t to develop power, but accuracy. It will definitely be difficult at first but really take time to master directing the position of your serve. Once you have that down you can begin to put some power behind it.
Missing your first serve isn't the end of the world. In fact, you should practice technique for your 2nd serve as well.
However, there are few things that are as unstoppable as a powerful and accurate first serve, which is why it's something you want to master.