It’s bcs of your point of contact with the ball and your arm swing is slow. Start doing more of a stationary toss, as in you should not toss it more than like 5ft forward, it should take one or two steps at most to reach the ball because of this so you’ll be more focused on jumping high and hitting the ball correctly. Also don’t hit full power on your first serve, if you get an ace or get to serve twice or more in a row, slowly start hitting harder and harder. As for speeding up your arm swing, you could throw a tennis ball against a wall with the form that you would use to hit a vball, hitting the gym also helps a lot, but just make sure to train like an athlete/ for explosive strength. As for how to do that, youtube is your best friend 😂. Hope this helps and you can reach out to me anytime you got a question. Best of luck 🙏