Slicing happens for a myriad of reasons, but it's only caused by one thing: If your clubface is open in relation to the path your club is traveling on at impact, the ball will curve away from the target. For right-handers, it's a left-to-right curve. Understanding how your forearms rotate in the golf swing, will help you square the clubface and improve your clubhead speed through the hitting area.

Get into your golf stance and hold a book as shown.
Rotate the forearms so that the book covers moves and your thumbs point up to the sky.
Complete your backswing and start your downswing.
As you come down, rotate the book back to the square position.
As you follow-through, rotate the forearms again so the thumbs point up to the sky on the release, then complete your swing.
Using a heavier book will help you develop strength in your forearms.