One way you can create more topspin on groundstrokes and more power on your serves or overheads is to prepare for the stroke (take the racquet back) with your dominant palm (that holds the racquet) facing down; or, as I like to call it to my students: “knuckles up!”
I do not, by any means, imply that this is the key to creating topspin or power on those strokes. It just helps.
Often, beginning players have a tendency to open the racquet face on the forehand ground-stroke (palm forward or up) backswing. That results in flat strokes or slice, and when they swing low to high the ball sails too deep.
Below you see two pictures – the wrong and the right way of taking the racquet back on the forehand groundstroke.
The same happens on the serve racquet take-back (regardless of your grip) – when the palm is facing up or to the side (see picture below), the wrist is stretched and that slows down your pronating action (at contact) very necessary to create power and spin.
The right way for you to take the racquet back when preparing for the serve would be to have your palm facing down (knuckles up) – see the picture below: