In the lower right hand corner of the game you'll see a ball with a green dot.
Kinda looks like this. This is normal spin. Your ball has an average trajectory and average roll.
Grab the green dot and move it upwards to impart topspin. Consequently, your ball will have a lower trajectory, will fly less and roll more if the ground is flat. Good in some situations, and not so good in others.
Grab the green dot and move it downwards to impart backspin. Your ball will have a higher trajectory, will fly more and roll less if the ground is flat.
Different shots will require varying degrees of topspin and backspin, but that part you gotta figure out on your own. :)