One of the biggest mistakes I see beginners making is that they rip their first serves out and in the net over and over again, then totally change their motion and dink in some 10% strength second serve. This is bad because they never get their first serve in and when they do get a second serve in, they’re as good as toast.
Always losing your 2nd serve points puts pressure on you as the server because you know you’re about to be attacked. Changing your motion screws up your timing on your first serve.
What is best to do rather than firing at 100% on the first serve and 20% on the second serve, keep the same motion for both, swinging at 70% speed hitting the same serve and banking on one going in. This way you stay aggressive, and yes you might double fault a bit, but eventually you will find a groove by keeping the same speed, rhythm, and motion.
When you become capable of consistently hitting slice and kick serves, these can be utilized on your second serve as the spin makes the ball drop into the court more consistently. They can also be used as first serves. You can also hit flatter harder serves as second serves. Being unpredictable is a huge advantage. But until you have that level of control over the ball, just focus on having one consistent motion where you swing at a speed that allows you to get the ball in 7 times out of 10.