Topping the ball:
One of the sources of "topping" the ball comes from inconsistent body height. That is, your ball to shoulder height changes throughout your swing. Commonly, you here instructors say "keep your head down." The idea behind that advice is to keep your body height level.
Often times, players adjust their body height--i.e. look up to early--in order to help the ball up. We see this with high-handicap players on fairways because players try to ensure that they hit the ball and send it flying. However, we don't want to hit at the ball, rather we want to swing through the ball.
One way to stop topping the ball:
A drill which might help to fix the tendency to lift the ball, or to adjust body height, is to hold your club across your shoulders with your arms crossed. Then, while practicing your body rotation, think about maintaining a level body height. It helps to have someone watching you as you do this. Have that person tell you when your body height changes significantly.
Hitting fat:
When players hit fat, the tendency is to take a divot behind the ball and to lift the ball into the air. Again, this happens because players are trying to help the ball up rather than swinging through the ball.
One way to stop hitting fat:
If you're practicing on a grass range (this is the best place to practice this drill because it is difficult to know and therefore to cure fat shots on range mats), then use aerosol chalk to spray a line extending through the middle of your stance at address. Your clubhead should rest along this line. Then, use your pitching wedge and try making some punch shots. You want to make a divot in front of that line. The goal is to keep that line intact. You don't want to get rid of it or hit behind it. In order to promote making a divot in front of that line, you want to think about proper weight transfer, i.e. shifting your weight from right to left as your make your through the downswing.
I hope this helps. Tell me if they work for you. These are just two tips I think are helpful in curing those problems. They certainly aren't the only ones.