Parenting your preschooler (read disciplining)

Gosh ... where to start? Really, I'm starting to think that all the parenting experts have no children.
I am the desperate mother of a spirited child. I have to admit it, I don't know how to discipline my kid. I have tried everything and nothing worked -- or it worked for just for a short time. So, tonight I'll take my daughter to taekwondo. People keeps telling me that martial arts are excellent for learning self-control, so I'm going to try this one too. We'll see how it goes.