There are a lot of different things that go into a good shot.
First and most importantly, you must hit the ball in the center of the racquet. Use your feet to get in a position to do this. As you get in position, make sure you have turned your shoulders and started the racquet back. As the swing starts use your shoulder rotation, your legs (weight going forward), and your arm in coordination to get all you can out of your shot. It may take some practice to develop the smoothness necessary to get all of these things working together.
A good tennis professional can help you by working on repetitive drills that develop timing and rhythm, which will coordinate all of your movements and help put it all together.
A ball machine or a wall can also improve your ability to make these things a habit.