How to improve my golf swing?

are you *always* topping the ball? or just often? if you're making exactly the same mistake again and again, stop practicing. you are creating bad muscle memory. get an instructor to show you what you're doing wrong, then practice a lot.

if you are only *often* getting it wrong, and sometimes getting it exactly right, then you just need practice.

generally topping is from looking up, taking eye off back edge of ball. try keeping head down for a count of 3 after hitting the ball. go to a range with a friend and you keep head down, don't even look at the ball, have the friend describe the flight of the ball. Lessons from a pro, always can help.

Think "What About Bob" - baby steps. Stand with the your feet inline with your target and the ball just inside your left (for a righty) heel. Start by taking quarter (or less) swings and just try hitting the ball straight. Once you get the hang of this, try to get replicate this several times. You should feel like your almost throwing the clubhead at the ball, allowing the weight of the club to provide much of the power of the swing. Once you feel comfortable with a quarter of half swing, gradually increase your backswing until you feel comfortable - the further you take the club past horizontal with the ground, the harder it usually is to control your swing so that is usually a good "top of the swing."

Topping the ball is often due to a reverse pivot. A "reverse pivot" is when you lean toward the target at the top of your backswing and lean away from the target at the finish.

I suspect you finish your swing on your back foot (right foot for right hander) instead of following through to your front foot. You want your weight over your back foot at the top of your swing and over your front foot at the finish. When you finish your swing you should be able to lift your back (right) foot and remain balanced.

I try to put my left shoulder over my right knee at the top of my swing. This ensures I get my weight back and also gives me a lot of torque (and more power). After impact, I let my right shoulder push my chin forward as I follow through.

This works well for me, but your mileage may vary.

Lessons, lessons, lessons. The best money you'll ever spend. Find a PGA pro and not just "a guy who knows stuff". Focus on what your pro tells you. Don't listen to friends or playing partners (or fluther posters).

A site called GolfLink has a lot of tips and video tips to help you improve your swing. They can even help you calculate your golf handicap.

2 sites you can try -
The simple golf swing
how to improve your golf swing