Think of all your shots as being graded as a "1"- Slow, "2"- Medium and "3"- Fast. This is concerning your actual racket head speed.
 
When to hit a "1" type ball? In warm up and when you are just practicing, working on style and form. More importantly, use the "1" type when your opponents hit his/her "3" - Fast shot.
 
The "2" type, medium, ball is used the most often, 80% of the time. At this pace you can keep pressure on your opponent yet not make many errors.
 
The "3" fast type ball is used sparingly. It is used to keep your opponent out of balance and to finish the point decisively. You will use this less than 20% of the time.
 
Concerning your serve. The greatest mistake is hitting your first serve as a "3" type ball, missing it, then hitting a "1" type second serve. This is how you lose. Hit your first serve as a "3" type and your second serve as a "2" type. All pros hit the first and second serves as a type "3."
 
In the rally, the mistake is receiving a "1" type ball and returning it with a "1" type ball. This allows your opponent to attack you with his "2" and "3" type balls. When you receive a "1" type ball go after it with a "2" or "3" type ball. You may not ever get a better chance to force (win) the point.
 
Good luck on the Court!