Hitting with topspin is a key to developing consistency and depth on your groundstrokes. To generate topspin, you want to brush up on the ball using a low to high swing path.
Place the ball between the net cord and your racquet. Then, using a low to high swing path, brush the ball off te net cord and over the net. Done correctly, you will see ball rotate forward with topspin.Adding topspin to your groundstrokes will give you greater margin of error.