The majority of groundstrokes should be hit with some topspin.  Topspin is the forward rolling spin you put on the ball.  A ball hit with topspin tends to rise quickly, continue to climb as it crosses the net, and then drops down sharply, bouncing higher that a ball hit flat or with backspin.   Most good players try to hit the ball with lots of topspin because it is a safer shot (it clears the net and tends to stay in the court); it allows them to swing hard without worrying about the ball sailing long; and it puts pressure on their opponent, as a ball jumping up forcefully around shoulder-level after bouncing is difficult to handle.

Topspin can be hit on the forehand or backhand side.  It can be hit as a groundstroke and it can even be hit out of the air.  The primary way to generate topspin is to have your racquet's swing path go from low to high from the beginning of your forward swing, to contact, and into the follow-through.   The best way to accomplish this is to bring your racquet low on the backswing, either by dropping your racquet head or bending your back knee or both.  You want the lowest part of your backswing to be lower than the point where you will make contact with the ball.   Swinging low to high from this low backswing will ensure good topspin on your shot.

If your contact point is just below your waist and you take the racquet straight back and down to knee level, and then swing so the end of your follow-through is up around your shoulder, you will produce plenty of topspin.

Some people mistakenly believe that they can get more topspin if they roll their wrist just as they make contact.  This usually results in netted balls.  The racquet face must be fairly vertical to the ground at the point of contact.  It is the physical fact that the racquet before contact came from a height below the ball that topspin is created.  Pros often look like they have rolled their wrists or twisted their arm and racquet over the ball, but super slow-motion video shows that this is not the case.  The racquet is almost vertical or tilted forward slightly. 

This false notion of turning or rolling the wrist also leads junior players to believe they can generate more topspin if they adjust their grip to a more extreme variation of the Semi-Western Grip.  While Semi-Western Grips can produce as much or more topspin as Eastern Forehand Grips, it is more a function of the swing path and point of contact that these grips force upon the player.  If adjustments for this are not made, those adopting a moderate or extreme Semi-Western will frequently top the ball and drive it into the net.  

Surprisingly, these players usually don't become frustrated by their lack of success and revert to a milder Semi-Western Grip or even a modified Eastern Grip.  Instead, they make their grip even more extreme and try to hit the ball harder.  This just insures more failure.

A good way to practice hitting a forehand groundstroke with topspin is to stand near the baseline and hit feeds to your self out of the air.  Toss the ball to about eye-level and hit it as it begins to fall into your hitting zone with a firm wrist and a low-to-high swing.  Once you can do this easily, hit some drop feeds to your self where you let the ball bounce first.  Use the same low-to-high swing.  Your shots should be clearing the net by several feet and landing deep near the opposite baseline.  If you are hitting with good topspin, you will see the ball bounce up sharply when it lands, a sign of heavy topspin.  Finally, get a hitting partner to toss or gently hit you some groundstrokes and apply the same low-to-high swing motion.  You can do this progression on the backhand side, too.

Keep practicing and soon you will be hitting all your groundstrokes with moderate to heavy topspin.