There are many different variables affecting your serve. However, the toss is the most important part of the serve. If the toss is consistent and dependable, your serve is likely to be the same. Timing and rhythm both play a big role in your ability to toss the ball well and to make contact at the optimum point.
Try using this simple counting idea when you are serving: when the ball and the racquet are at the starting point of your motion, say one.
As your non-dominant arm moves upward to toss the ball, say two.
Finally, when the racquet meets the ball, say three.
Initially, I would recommend counting out loud so that you can hear the speed that everything is happening. The tendency is to go too quickly through the motion and to the point of contact, so try to emphasize keeping the counting slow and smooth.
Each server will have a slightly different timing when serving, but the time between the numbers should be the same (consistent). Eventually you should be able to just count to yourself, while focusing on relaxing your arms and keeping your eyes on the ball all the way through the point of contact.
This simple idea will help you develop a reliable first and second serve, and start points better which will help you win more tennis matches.