Problem: A slice indicates that your clubface is open relative to your swing path. In most cases it also means that your swing path is too far down and to the left (for a right handed player). When you put these swing characteristics together, you will lose both distance and control off the tee.

How do we fix a slice? 
Again, assuming you are a right handed player, we need to set the stage so your swing path can move further to the right, and the player needs to learn how to square or even slightly close the clubface at impact.  These conditions will eliminate that high and weak ball flight to the right.

Try this drill: At setup, turn your front toe slightly in. With your trail leg, flare that foot even further away from the target (shown in the first photo). Then with the club across your chest, feel what it is like to turn your shoulders behind the ball (shown in the second photo). In a proper full backswing position (for a right handed player), you want to feel your weight working into the inside part of your right heal. After practicing with the club across your chest, do this same move in your golf swing.

Why: If you tend to slice, adjusting your foot position like this will set you up for a full and unrestricted turn behind the ball. Doing so will aid you in your pursuit to move your swing path further to the right, while giving you more time to square the clubface coming into impact.