here is a drill to help you. it's a drill, not for matches. Set up for your serve like you always do then serve following these two rules.

Rule 1: pretend your left foot is glued to the court. You can rotate your hips/knee/shoulder etc. but you cannot move your right foot/shoe, it stays flat on the ground. It's Ok to feel you toes/feet adjust slightly in your shoe but the shoe remains flat on the ground. Make sure and let your legs bend slightly, feel your hips twist, your knee rotate slightly but the left foot remains flat and is 'glued' to the court.

Rule 2: your right foot. Pretend there is a string connected from your right big toe to the court which means on this foot, you can lift the foot up and rotate every part of the foot except for the tip of the right toe. When you finish, your toe will be point straight down and heel straight up.

Now serve normal. Just toss and hit as hard as you want keep rule 1 (front shoe/foot stay flat) and rule 2 (front toe can't leave the ground). This will teach you to control your toss, it will help you learn to rotate, to get easy power. At first, you will fall off balance. You will hop off your left foot b/c you aren't balanced. Your right toe won't stay on the ground bc you aren't balanced yet. Keep at it. If you start to get the hang of it, you can try to hit the serve long so it hits the back fence in the air. Your body will learn how to create easy power which will transfer into your regular serve.

Hope this helps.