The Topspin Forehand
OK lets learn a topspin forehand, (keep in mind that this advice is for a right handed player so if you’re left handed just change everything to the other side) first of all get used to putting spin on the ball by hitting it with an upward or downward slicing motion, this way you can get used to putting top spin and back spin on the ball. After you’ve gotten used to the idea of putting spin on the ball try going back to the forehand topspin.
To get this shot right you’ll have to hit the ball on an upwards angle, taking your stroke from the right hand side down at about elbow level to the left hand side up over your shoulder, your hand should be in front of you when you make contact with the ball. As you hit the ball your hand should be turning over from right to left, anticlockwise.
In the beginning get used to hitting the ball as it comes down as that will put extra topspin on it and get you used to the forehand topspin, then when you’re more comfortable with it you can start taking the ball on the rise as well.
Here is a great link to learning other strokes in the game
Strokes of Table Tennis
Return from Topspin Forehand to Playing Table Tennis