Try starting out at the ten foot line , serving over handed. Assuming you are right handed, ball in left hand,held out in front of right shoulder.Right foot back, left foot up.
Controlled toss, reach and contact the ball high. You should hit the middle of the ball with the middle of your hand-
Once you get it over 10-15 times from this distance--back up a few steps and repeat.
Repeat until you are consistently serving the over the net--be sure and use an open hand--not a fist so you can master serving to spots eventually.