It's all about club face ladies and gentlemen. The ball almost always goes in the direction that your club face is looking at impact. 

If you miss to the right or slice the ball all the time, you're leaving the club face open. If you hook or miss left, the club is looking left at impact. It's that simple. 

In terms of working the ball on purpose, you need to think about your swing path to get it started in the right direction. Basically, you need to take the direction you want the ball to start it's curve, then double that angle...and swing in that direction. 

So, if you want to hit a 10-yard slice, you need to set the club face to look at your target, then swing 20-yards to the left of the target. 

Hope this one helps clear up some misconceptions.