I started out as a soldier and pyro, simply because those classes are pretty easy, but before you actually know what is good you just have to play a lot and you will see it.
Also try to look at what classes usually kill you, and why they kill you, you can then develop some kind of tactics against those attacks.
Selected your class, try to be the medic of one of the best players of that class, just looking at their style gives you so much information how you should play the class, then when you practise a lot and a lot you will be able to create your own style.